Maokai is a good pick vs high poke, high cooldown bursty mages, as he is tough enough to survive their burst, can outsustain their poke, and can jump on immediately jump on them with twisted advance after they cast a spell, easily winning the trades.
Sorry, this isn't a complete list, there's like 100 champions so it's hard to do a full write up for everyone, or even get experience laning vs everyone, but most champions are similar enough to the ones already listed that you should be able to make a good guess at how the fights will go.
Champions you beat:
Maokai is a great counter to twisted fate. Twisted fate can't do anything to you unless he has a yellow card up, so once he uses that, or before he pulls a card at all, jump on him with E -> W -> Q and then run out of range before he can pick a card. You'll win the trade everytime, and chunk about half his hp in the process. You also have far more sustain than him and if you're using my build enough magic resist to pretty much ignore him. You can force him to turret hug all day and he'll try to farm minions with wild cards from long range, which are tricky to last hit more than one minion with; there's not much else he can do vs you. While he's at his turret, poke him with saplings.
you can outsustain her and win trades easily, and interrupt her ultimate with Q. Pretty easy lane.
if you see them start to cast a skillshot you, W them. That makes you untargetable, and now their main damage ability is on cooldown, so you win the trade. With your high magic resist build and your passive you should be able to outsustain their mana bar.
If your jungler has any clue he should be ganking the hell out of this guy because he's free gold with maokai opposite him. Just lock him down and burst him down together with your jungler, you should have enough CC for an easy kill. This guy has zero escapes and zero CC, and has to push to get his shield. Free gold for junglers. If he doesn't push the lane he gets no shield, so if he's turret hugging because he's scared you can just harass him down with saplings at his turret and win the lane easy.
If the jungler doesn't interfere though, this is a roughly even lane, although you still have an advantage IMO because you can poke him pretty well with saplings.
She'll have a hard time killing you if you're using my magic resist build, you've got 90 mr with chalice. You're also a pretty good pusher, and if you thought leblanc had trouble last hitting at a turret before, try last hitting at turret while dodging saplings chunking you for a quarter or more of your health.
Leblanc has no sustain, but watch out for the jungler, you don't want to get her fed. I recommend buying wards on the same back you buy your chalice. I also recommend maybe buying a couple health potions on top of the crystalline flask, so she can't just whittle you down with autos or Q -> ult spam everytime it comes off of cooldown.
Keep in mind: If you go even with leblanc, you still win, because you're more useful late game, and you can CS better. So even though you have the potential to shut her down completely, she also has the potential to kill you, she IS leblanc, so don't get cocky. I recommend playing this somewhat passively, especially from level 6 on, just because her burst is insane. You should ward and shouldn't take too many risks as her burst + a well timed jungler gank may get a kill to turn the lane in her favor. If she doesn't kill you, you win, because you can CS and push better and you're more useful late game. If this lane goes badly, keep in mind that you can walk around and into the side bushes and farm with saplings from there. The bushes will make you untargetable and harder to kill, unless she has a ward. That works for any lane that goes to hell vs opponents with targetted skills, btw, not just leblanc.
Champions who you don't beat, and who you probably aren't a good pick versus:
Maokai has no hard counters in mid lane, in that there's nobody who can reliably kill him if he plays passively and intelligently or force him to lose significant amounts of farm. The short length of the mid lane makes it super easy for maokai to just farm with saplings and push the lane repeatedly even if he's forced into a defensive posture.
However, he does have some bad matchups in the sense that he typically goes even in farm with some hard-carry type mages who are more useful than him lategame. With that in mind, I recommend not picking maokai vs Ryze or Karthus or Kog'maw, or if you suspect you will be laning against an AD Champion:
Maokai may have the slight advantage in this matchup, as you can push and force him to last hit at tower, but it's not a big enough advantage that you're really a counterpick, and ryze has a really strong late game to make up for any slightly unfavorable matchups. Jumping on him is dangerous as his cooldowns are low and his sustained damage is high, so you probably can't win trades with his health bar, although it depends what runes he's using... some ryzes are softer than others, esp the ones who use move speed quints. But once you're done comboing him he'll just spam you with spells, and possibly do more damage than you did to him. You may run him out of mana, if you do you can zone him. But once he gets tear or blue buff it's very hard to run him out of mana or push him out of lane, and this turns into a farm lane, which is a bad thing because ryze is more useful than you are if it goes to late game. If it goes this way, you're best strat is probably to just push the lane hard repeatedly and try ganking the hell out of side lanes. If you can get your bot lane and top lanes fed, you might be able to end the game or gain an insurmountable advantage before late comes around, at which point ryze will be a serious problem.
Jumping on this guy is dangerous as he'll wall you in to slow you, then spam Q and AOE. Since he'll know your exact position after twisted advance (right on top of him) it's an easy to land wall. You may not win that trade unless your jungler jumps into the fray after you, which wouldn't be a bad idea, karth is not be that hard to kill with jungler help.
When running away from karthus after your spells are used, keep in mind that it's more important to dodge his skittles than to get out of range of his AOE... his AOE is not mana efficient at all.
You can farm fine opposite him, due to your passive if he tries to heavy harass he'll end up just healing you. Just try to avoid getting hit by his wall, that makes it easier for him to land Q. The problem is you have no reliable way of stopping him from farming which is a problem because a farmed karth can cause trouble for your team late. That being said, If you find yourself facing karthus, I recommend playing agressive on karth and trying to freeze the lane, and hopefully your jungler will help you kill him. You should probably take ignite if you know you're facing karthus. There's no particular reason to push vs karth as he doesn't roam that often (He's squishy, can be caught in jungle, and he typically just ganks with ult), and he can CS at tower very easily. So I would recommend taking potshots at him with saplings and trying to last hit with arc smash/autos. If you can chunk him with a good sapling toss, you may get him low enough to jump on him and kill him when the next sapling comes off of cooldown. Definately buy wards though, you want to know where the enemy jungler is.
Cho'gath is another one of those tanky midlaners that nobody plays for some reason but is actually a really solid mid laner.
This guy is just annoying for maokai, mostly because jumping on him is a really, really bad idea, because that telegraphs your position for a very easy to land rupture/silence combo. Rupture counters twisted advance.
If you try to twisted advance him, you'll be slowed and silenced and knocked up, you'll eat a ton of damage and you're not going to do anything to him anyway. He can't really kill you either unless you play stupid or the jungler comes when you're out of position, but you're probably going to get out cs'd as his auto attack is better than yours with his vorp spikes, and you can't really fight him very well. Just try to last hit as best as you can with saplings and arcane smash, and auto attacks if possible. Whatever you do don't cast twisted advance on him; vs cho I wouldn't even take twisted until level 4, arc smash is more useful, cause if he starts attacking you in melee you can use that as a disengage.
The only upside is that you're a more reliable side lane ganker, as cho'gath's ganks are sortof wonky and easy to avoid, so even if you get outcs'd by a bit, hopefully you can make up for it with superior side lane ganks in the mid game.
I've yet to fight this guy as mid-lane Maokai, but I see him all the time when I play jungle Maokai and he's super-annoying. I think picking mid-lane Maokai vs Kog would be a really bad idea, you'd probably rather have some kind of assassin who could dive him and kill him, or Ahri. From my experience fighting him late game as Jungle Maokai, I don't think Maokai really has the right kit to deal with him late game.
As for laning vs him: dunno, but you'd definately need to juke well because his ult is stupid. Make sure to have boots before he hits level 6.
Notable roughly even Matchups:
The majority of the cast is in this category, Maokai is a fairly safe mid pick and goes about even with most common mids.
She has surprisngly high sustained damage with her foxfire, jumping on her is actually somewhat dangerous esp once she gets ult. If she whiff's her charm or just used some abilities to farm, go ahead though. She's hard to gank due to her ult and she can chase you down well with it. I recommend inviting your jungler to gank her to hell sometime before level 6, and if you get a kill on her you have the potential to shut her down. Otherwise it's a farm lane.
You can't jump on her safely, the fang spam is too strong, although if you see her fang something that's not poisoned, go ahead, you have five seconds to unload then leave.
Keep in mind, there's no reason to be facing her when comboing her. just twisted advance, move slightly in any direction, toss a sapling right on top of yourself, and arc smash (can hit targets behind you, FYI). You'll want to avoid facing her as her ult stun is annoying, and only stuns you if you're looking in her direction. There's no reason to be looking in her direction.
You're perfectly capable of just farming though, which is how this lane normally turns out; she casts a lot of spells so you get lots of health back from your passive, you cast a lot of saplings and kill a lot of creeps with them, and nothing really happens.
The quintissential farm lane. There's pretty much nothing you can do to her with her black shield and high spell vamp, and it's not like she's going to kill you either unless you're at 500 ping and she lands every binding. You're both going to just be pushing the lane and then trying to gank side lanes, so try to do a better job of ganking/counterganking than she does.
I do not recommend jumping on this guy too agressively because if he has quick reflexes, he'll use his dash while you're mid flight then ult you into his tower. Twisted advance will follow people through dashes and flash, so you can find yourself completely out of position. Very dangerous, that can really ruin your day. although if he m oves in to last hit with auto attack and is nowhere near his tower, you can go ahead and jump him.
He's also less squishy than most poke mages so he's hard to burst down, this is typically just a farm lane, although maokai may have a slight advantage. He is melee and has no sustained damage, so he can't really do anything to you, either.
This guy gets armor from his lightning rush, and move speed, and can combo a stun from it if he has enough energy, so if you jump on him he's likely to stun you to mess up your combo and run away dodging your sapling. With that in mind, i recommend E --> W --> Q for this guy, W --> E --> Q is harder to land on this guy than most.
also his auto attack is more annoying than most mages, so he can harass you pretty well. It depends on the players but this is usually a farm lane.
Sometimes they send their AD mid, especially once they realize you have so much magic resist. These tend to be the worst kinds of matchups for maokai due to the annoying auto attack harass, but you should be able to hold the lane and go about even on farm if you play it right.
You can free farm from range with saplings (much easier past level 7). Just farm with saplings and ignore them, or if they're a squishy ranged carry, you might be able to jump on them and kill them, but try not to take too much damage from auto attacks.
You can't trade with most bruisers just by jumping on them, as bruisers are usually good at dueling, but that's not a problem, just harass and farm with saplings, if they jump you you can peel them off of you and kite with arc smash, and since it's mid, you're too close to your tower for them to follow up on that much, so you win the trade. I recommend building ninja tabi if you're laning against AD, and skipping chalice, and rushing shurelya's.