I feel like this warranted a separate chapter by itself because many times, people on your team do not know what to do when they see a Jayce mid. Even at Diamond elo, too many times we have lost because we had 5 AD champions and the other team built all armour.
As a squishy AD champion mid, ideally your top lane should either be a tanky AP champion or a champion with true damage. Your jungler should either be AP or tanky as well. Note that the mechanics of team composition is pretty advanced, and there is no way I could cover everything in one chapter. Everything I write below does not take into account of your bot lane composition, nor the enemy team composition. It also does not take into account what your team wants to do (poke, all-in, split push, protect the carry, etc).
These are simply champions that work well with Jayce at the Diamond elo level of play.
Good Top Laners:
rating: 3/5 - Her AP burst forces the other team to build magic resist, preventing them from stacking armour. Overall decent champion.
rating: 5/5 - I love Cho Gath. He serves as a tanky frontline to protect you as well as doing ability power damage AND a true damage ultimate. Also has tons of crowd control.
rating: 4/5 - Almost as good as Cho Gath. He's a tanky frontline as well, but lacks the amount of CC (crowd control) that Cho has. However, Darius offers more in terms of damage. Especially, his chain ultimates. Fun stuff.
rating: 4/5 - Better than Akali because she has some crowd control. Overall decent pick.
rating: 3/5 - Overall decent pick. This is only a 3 because Elise does nothing to protect you.
rating: 2/5 - I really don't like Fizz. The enemy team usually just ignores them and dives onto me. Only mentioned here because he's AP and a lot of people are currently playing him at top lane.
rating: 5/5 - Good Kayles are amazing. Their damage, their heal, their 40 second late game ultimate.
rating: 3.5/5 - Good for his ultimate and his CC. If Kennen builds tanky, my rating for him jumps to 4.5.
rating: 10/5 - BEST CHAMPION EVER TO BE WITH JAYCE. I have almost never lost a game where a Nidalee was on my team (something like 15-1 as of March 5, 2013). The Nidalee and Jayce poke simply has no counter in solo queue. Both AD and AP works.
rating: 4/5 - Good because Rumbles naturally build tanky. His damage is also nothing to laugh at.
rating: 4/5 - Tanky, consistant AP damage. People also tend to chase singeds for some reason. What is there not to like?
rating: 3/5 - A decent champion because he is tanky and offers AP damage.
If your top laner is an AD champion, your jungler should ideally be an AP champ. Otherwise your team will have a tough time late game.
rating 5/5: Tanky with an AOE CC that stops them from moving. Allows you to do more damage to a stationary target. I love mumu junglers.
rating 5/5: Same reasoning as a top lane Chogath
rating 4/5: Same reasoning as top lane Diana
rating 5/5: I actually really like Evelynn junglers. With her ultimate, there is no way for any champion to out run Jayce/Eve when she ganks. Shoutout to Kelberry's Evelynn, never lost a game with him as Eve on my team yet (4-0 and counting).
rating 5/5: Very very strong presence mid/late game with his ultimate.
rating 3/5: He has very weak ganks early game, when Jayce needs help the most. But Hecarim has a large impact late game, which is why he gets a 3 and not a 1.
rating 4/5: Very strong ganks, and tanky as well.
rating 4/5: A good Karthus jungler is nothing to laugh at. He has good ganks with his wall thing, that slows. Karthus has a very large impact late game as well.
rating 5/5: Very strong ganks and initiates with his ultimate. Extremely tanky as well.
rating 5/5: One of top junglers I like to have on my team. His ganks are amazing pre-6.
rating 4/5: A good Nautilus is hard to come by. But he offers CC, tankiness, and very strong ganks.
rating 3/5: This one is a bit iffy for me. Honestly depends on the Shaco.
rating 3/5: Very weak ganks pre-6. And even when he hits level 6, Skarner does very little damage. Only useful because of his grab, it's game changing.
rating 3.5/5: Good during the late game and against teams that cannot kite him. Early game, however, it's hard for an Udyr to successfully land his Bear stance.
rating 4/5: Very strong ganks, as well as an ultimate with a knockup. Furthermore, she is tanky and does a lot of damage. All in all, a strong pick.
rating 5/5: Another one of those underplayed champions that I really like. He has extremely strong early game ganks and damage.
rating 5/5: Strong early game ganks that does not fall off in the late game. He's kinda broken at the moment.