Singed is probably the most fun character in the game in my opinion. Being able to get kills/assists while running around fairly recklessly and still being able to get away is awesome. Not to mention you can throw people over walls, into towers and into your team.

For fun.....Ziggs IMO. Bouncing bombs, mine fields, HUGE Explosions... crazy laugh... i have fun on him even if im not winning.

Singed, Olaf and Veigar are immensely fun.

I find Udyr fun because of his "Stance Dancing" mechanic.

Fun Champ? Singed. Nothing more fun than that fling.

well, i really like gragas. He is awesome at solo lane. SUPER FUN!!!!

Anivia's a very unique champion because of her wall, and I find that nothing makes me giggle with glee quite like a perfectly placed wall, or a terribly placed wall that my team mates manage to turn into an epic wall (Singed Flash over wall and tosses the enemy AD carry back over the wall to the 3 DPS + 1 Support champ drooling over the fresh meat while Singed uses Hourglass while he's still stuck with the other 4 enemies on the other side of the wall).

I'm surprised no one mention Cho'Gath. He is the champ I have the most fun playing.

Yi.

I find one of the new ones, Sejuani, as a really fun tank to play, if you know how to. Tank+chaser, really annoying and really tough with the right equip.