Kenny, definitely. He can die constantly and come back to life, not to mention it is quite funny when the creators use him to slip swear words past the television censors...
It's funny how they've turned Stewie from this raving lunatic bent on world domination, into a homosexual character on FG.
Anywho, my favourite character would have to be Kenny. It's just funny the many ways in which he is killed, and simply comes back to life. He plays a pivotal role in the gang, and you just feel sorry for the little dude once in a while.
I like when he is trying to steal a brownie and Marge cuts off his thumb. "Around the back! OOO, he's reaching around the other side! Fakout! AHH MY THUMB!" :P