Since, even though I've probably mastered mid the most, I play all lanes depending on what my team needs, I'll give a champion for each role.
Top Probably
[Renekton. I picked him up recently, and has been the first top laner that I can really connect to even after years of playing LoL. He has great sustain and can be built tanky if you don't have any other tanks or front liners on your team, or if you want more damage you can build that too. He just jumps into a team and stuns the carries and does AoE to everyone. It also helps that he doesn't use mana as after playing so many games as Katarina I'm used to spamming abilities. Highly recommended!
Mid This one is kind of tough but I have to say
Katarina. She is just so fun. Her kit works well with a great initiator to jump in and get the kill(s), and then her passive refreshes your cooldowns so you can quickly just jump back out to a minion or a ward. She is especially fun with certain junglers like J4 than can ult letting you use your ultimate on them for the full duration and can throw flags down that you can jump to, or Elise can has a nice stun that also lets you get a free ultimate.
Support This is a role I haven't played much lately just because the fact that I can't carry frustrates me, but when I get a good ADC I don't mind it. I usually go with aggressive supports like
Thresh and Leona as they can pretty much insure a kill every initiation.
ADCMARKSMAN SORRY RIOT In this role I usually go with
Lucian. I love him because his kit synergizes with itself so well, and gives you the ability to hard initiate, poke, or kite! His passive is his bread n' butter and allows for a lot of DPS when you use your abilities, and his ultimate lets you catch fleeing enemies like Caitlyn's or Jinx's but it can also be used when the enemies aren't fleeing so it gives it more utility. I would like Jinx but if she doesn't get feed I've found her pretty useless, and I just
hate, hate, hate her lore, and her persona... I feel like she should just be removed from the game because she doesn't really fit in with other champions in the League. I'm also kind of getting tired of these champions with global projectiles...
Jungle heimerdinger my favorite is Elise. She brings tons of utility to the team (she has 7 abilities) and provides wonderful ganks that synergize with a lot of champions (see above comments about Katarina). She gets in quick, chunks a bunch of damage, and stuns them. If played properly you can provide pretty high success rate ganks.
TL;DR