What I would consider the meat and potatoes of the guide. Renektons item build can change drastically from game to game, and once you get some experience with him, you'll be able to make your own judgement calls. For now, you can refer to the item in question and see what I think of it here.
Doran's items are awesome. They give you great early game stats, and the damage block on Doran's Shield specifically is fantastic against autoattackers. It's not uncommon for a good Renekton build to have at least 1 or 2 Doran's items to help you reach your other, more important purchases. They do have their downsides, however. In the end, they do they delay your more core items, and they start to become a bit of a gold sink if you buy too many.
A great item early game that offers everything a Renekton player wants: AD, flat Armor Pen, and Cooldown Reduction. I like to sit on Brutalizer unless my team could make good use out of an early Black Cleaver. The armor shred synergizes quite well with your E and could potential affect the entire enemy team. It's not entirely necessary to pick up a Brutalizer if you don't need your cooldowns to compete in lane. I would highly advise getting one against champions who have superior cooldowns though, like Riven.
While Frozen Mallet is good on Renekton, I would say it's just a tad expensive for the stats it gives you. You should be looking at the other Giants Belt items instead unless you absolutely need the guaranteed slow. At the end of the day, Frozen Mallet is a utility item that fits into a very niche role.
If there ever was a core item on Renekton, this is about as close as you can get. Sunfire really gives you everything you need. It's fantastic against AD tops because of the health and armor, and the passive magic damage single-handedly wins trades. It helps you push out creep waves faster, and it gives you another source of damage that isn't physical, making it harder to itemize against you. It really is one of the best items Renekton can buy.
Warmogs has seen a bit of a drop in popularity, but I still think it's a good item in certain situations. I like building it specifically when the enemy team is low damage or has a very balanced mix of Physical and Magical damage, simply because the best counter to both types is HP. You have to be careful though - if you suspect the enemy team is building multiple BoRKs or they have multiple %HP abilities, I would recommend buying something else.
Atma's is pretty bad. If you need armor there are plenty of other viable options, and if you need damage, well, there are other options for that too. "But Rusty, that's not the point. It gives you a bit of both in one item slot!" Yea, you know what else does that? Sunfire Cape. Of course Atma's gives you a bit of crit chance, but considering Renekton doesn't make the best use of that stat, it's almost useless to us. Basically, Sunfire does everything Atma's does but better. It's kind of in a bad spot right now.
An armor item that offers a lot of stats, Randuins is extremely beneficial on Renekton, and synergizes with his kit well. He can double-dash in to get to the enemy AD, use the active, and lower the DPS of that AD dramatically. It's also difficult to kite a Randuins owner due to the small passive slow, making autoattackers even more miserable. It has an awesome build-up as well, so if you bought that early Giants Belt but changed your mind about the Warmogs or Sunfire or FM, you can upgrade to Randuins instead.
Now that the mana has been removed, Banshees is actually a viable MR item on Renekton. Its buildup is great with the addition of Spectre's Cowl, and both the MR and HP were buffed. However, it's difficult to justify buying Banshees over Spirit Visage because of how ridiculously good SV is right now. Unless you really need to block that gamechanging ability, the smarter route is to go with Visage instead.
Thornmail is totally underrated, and it's in my opinion that Thornmail is actually the best late game Armor item. Essentially, it cancels out the enemy ADC's lifesteal, and during late game teamfights this is huge. Just imagine if you're able to reach the ADC and they can't sustain through your damage. It forces them to either recall or run away and find something they can actually lifesteal from, both of which result in an easy teamfight win.
If you're a high priority target, or you need to dive into the back line to fight a carry, this is a good route to take. If you can get this item before 30 minutes where teamfights really start to happen, you should have an advantage because their carries are forced to either engage you or run away from the fight. GA is also gamechanging in the late game when respawn timers are so long or the next fight will determine who wins and who loses.
This is an essential pickup if the enemy team is stacking armor. Due to how Armor Pen works in S3, you're able to penetrate an immense amount of armor combined with flat armor pen. If you've got a balanced build, you should be able to soak some decent damage while dealing decent damage, and as Renekton that's your goal.
This is Bulwark's "replacement" if you will. It fills the same role as Bulwark, but now I feel like buying Locket is entirely dependent on either your Support or very team-oriented Jungler. Your item slots should be dedicated to more selfish items. The only time I would ever consider buying this item is when I'm so far behind that I'm not even relevant anymore. You can either splitpush your way back into relevancy, or you can buy a Locket and group with your team in hopes of them carrying you.
It's hard to understand why more players don't take advantage of this ridiculously strong early/mid-game item. It gives some nice AD for the cost and the shield makes it really hard to burst you down in a full combo. I feel like Hexdrinker is a requirement when laning against the likes of Rumble or Elise. Maw is now a powerful upgrade to Hexdrinker, but it's definitely not required immediately.
One of the best MR items this season, if not the best item period. In fact, it's so good that I think it's a bit overpowered. It gives 20% CDR, which is insanely good on Renekton, and the passive allows you to sustain through most magic damage harass. Like I mentioned in my review of Banshee's Veil, Spirit Visage also has a great buildup, so you can feel comfortable building towards it very early in the game. Late game your empowered heals in teamfights are pretty sustain, and overall, I would say this item is almost as close to core as Sunfire.
This item not only gives you a lot of damage to work with, but it grants you some additional utility and sticking power. It can also burst heal you in a dire situation. Keep in mind that this does, in fact, work with your W. It doesn't give that much AD and the attack speed isn't usually a desired stat, but the other stats are great. I'm beginning to like BorK more and more.
I think Ravenous Hydra is the best DPS item you can get on Renekton. Think of it like an offensive version of Sunfire Cape. It allows you to push waves instantly once you group them up, it gives you more AoE damage, the health regen is pretty nice in lane, and it can cancel your W animation. You can also just sit on the Tiamat without going the distance to upgrade it, which is nice considering you can use the 1400g elsewhere if you choose to do so.
Bloodthirster is a great item if your team needs you to perform a more bruiser/assassin type role where you need to focus a specific target. It makes you quite strong in lane and you're able to farm stacks quickly. Renekton's scaling is actually pretty good, so the 100 AD is fantastic. BT combined with 1 or 2 mildly defensive items makes you a mid-game monster where AD carries don't really have the damage to punish you for such an offensive build. Late game BT becomes problematic when you're more prone to dying in teamfights. Building stacks isn't always an option, at which point something like a Hydra would be better.
Bad item Renekton. Attackspeed is one of the least beneficial items on him and Wit's End doesn't have the best synergy with Renekton as a champion.
Ok guys, let me drop some knowledge onto you about Trinity Force. I will admit, it's an ok item on Renekton. The sheen procs, the HP, the movespeed, the chasing ability - all the stats, minus the mana, are useable in some form or another. But if mid-game rolls around and all you have to show for it are some dorans blades and a TriForce, then your team is going to get rolled in skirmishes. The items I suggested are much better at fulfilling the role your team needs mid game. TriForce is cool, but you just dropped 4K gold for something that offers very little survivability in terms of HP and resistances. Just because Dominus gives you bonus HP doesn't mean you can build something as purely offensive as Trinity Force is. At the end of the day, Trinity is a #ForeverLuxury item.
The jungling items are kind of peculiar. I wouldn't recommend jungling Renekton, but I have seen some rare cases in which people got away with building them in lane. Elder Lizard has been nerfed too many times to be completely viable on laners, Wriggles is just bad, and I'd rather you just get the Tenacity from Merc Treads instead of buying Ancient Golem. The only scenario in which I can see Renekton getting away with Ancient Golem is when he desperately needs both Ninja Tabi and some form of Tenacity.