i'd have to say that i've had a lot of fun with cho, fiddles, and urgot, although i'll admit i haven't played a lot of champions yet. i enjoyed the few games i've played with bowserclone, too, but i didn't play him enough to really get the hang of him.
i tend to like specific things that characters do, as opposed to the whole of the character. i love ashe's ult - sniping across the map is awesome. i love using cho to eat the dragon and baron, and the sound effect is awesome. i love ganking with someone like fiddlesticks who can do such crazy AOE damage and can permacast drain when he has bluebuff. i love rammus's voice, as it's the least annoying one in the game. i love that, with the right runes, a build that's got one or two relatively common mana regen buff items, and bluebuff, you can continually cast kog'maw's ultimate move without running out of mana, ever. i love sona and janna's support when i'm playing cho =)
i hate annie, shooty girl, and ryze, sooooo much. i've died more to those three characters than any other five in the game. and i hate taric for his stun move, which is so durn long that it feels like an eternity.