Gil Gunderson. There is something amazing about a down on his luck salesman, who should probably not be a salesman to begin with. I don't know what it is, but I find myself laughing hard whenever he starts rambling into a self-depricating sales pitch.
Jeremy Freedman. So it goes without saying that I also love Pimply-faced Teen. "If I had a girlfriend, she'd kill me." I don't know why, but this character also cracks me up...did you know his mom is Lunch Lady Doris? Yeah, neither did I.
Cletus Spuckler. Red-neckiness is timeless. Whether it's the scenes from the parenting class or giving a day off to his pig instead of his wife. Cletus is everything I strive to avoid. He's like everything I ever needed from King of the Hill, but never got.
I guess what I love about these characters, is that even in the less than great seasons, their schtick still runs true. The deviation in hilarity mainly fell on the title characters of the Simpson's running off track of earlier, funnier characterizations, as well as the humor becoming more contemporary and topical. They also fell to making whole episodes where the jokes were all just "what if everything was Irish, or in space, or the Harlem Globetrotters came to the island." Still I have great love for the Simpsons, and even there worst seasons are still better than some of the middling seasons other sitcoms have produced.
One of my favorite episodes has to be 22 Short Films About Springfield. Modeled after Pulp Fiction and filled with some of the most amazing moments of Season 7. I love the side characters of the Simpson's and this episode delivered story after story. Each written to deliver a quick gag.
Marge vs. The Monorail is also great, Wes. Lyle Lanley is amazing and that also reminds me of how Phil Hartman's death greatly impacted the series as a whole when they retired most of his characters.