Calvin's creator is still among us. (H/T.)
Noted lots of love for Pearls Before Swine in the comments. I can't understand why; worst comic I've ever read. I mean, come on, I get satire, but mean-spiritedness is not my cup of tea. Get Fuzzy was also mentioned; a formerly-good comic that has degenerated into "what will the cat beat up/destroy today".
One that wasn't mentioned: Candorville. May I be frank? Right down there with Pearls. Awful, awful, awful.