There may be others I’m not thinking of, but in any case the finalists would have to be some ranking of Moe, Moleman, and Gil. Yet who is the most truly pathetic? (Comic Book Guy is pathetic, too, but he’s so arrogant most of the time that his patheticness is less funny.)
Moe, of the three, most often takes it too far, as he has tried to commit suicide on multiple occasions. Gil’s bizarre level of optimism in the face of his patheticness is a key part of his comedic value — one might say it’s all of his comedic value. Moleman is an old guy with bad eyesight who is remarkably well fleshed out; more so than Gil, at any rate. His value is being overlooked. “I was saying Boo-urns” is a classic line that doesn’t work if anybody else says it.
I’ve have to rank them, from least to most, as Moleman, Gil, and Moe.
I’m willing to listen to arguments, though.
36 Chambers - The Legendary Journeys: Execution to the max! 
“This is Moleman in the Mornin’. Good Moleman to you.”
Yeah, Moleman is probably the character most worthy of our pathos, at least in the Good Era of the Simpsons (approximately seasons 2-10, although there’s a big dropoff after 8).