I'll go position wise.
Jungle: I really enjoy Darius in the jungle, he's an all around balanced jungler. He has decent ganking, moderate speed, incredibly hard to invade, can duel very well early, has a wide variety of builds/transitions well.
Top: Rengar, he is just so hard to budge out of lane, you can pretty much farm all day and have free harass with you abilities. Granted pre 6 he has no real escapes, but his mobility through the bushes makes it really hard for a jungler/top laner to coordinate a gank. Also, there is a reason so many teams picked him up for a solo laner in ipl, his ability to farm under a turret is hilariously easy.
Mid: I still prefer the old school mid laners, I know the meta really doesn't fit them well anymore but my heart goes out to Malzahar. You toss an E out into the minon wave and you can go do other things while your ability clears out the minons, his ult is a great stun/suppress to remove someone from a team fight and/or grab them out of position;and he is one of those few APs that you can build tanky and still get away with it.
AD carry: My least liked position, but I always go with Ezreal, he is just such a safe pick, he has an escape, and his early/mid game numbers are so good that very enemy carries can actually engage in a full out fight with him and expect to win. Also I love that he doesn't have to go the traditional AD carry route for items, trinity force is just all around fun and lets you snowball hard.
Support: I like Nunu, a ranged slow to constantly harass the enemy, you can constantly trade off with an enemy and come out on top because a simple Q will bring up your health. Unlike some other supports who fall off late game, Nunu's W is constantly good for the carry and his item build is very flexible. Also, how can you hate a champ that made the bottom lane empire move.
As a side note, I'm amazed at the ammount of people who said Garen, the poor guy got hurt so hard by that update late summer. Just shocked that people still play him, he had a hard enough time to maintain a position in games before, and the update just hit him like a rock.