Best:
Classic period Peanuts, from the 1950s to whenever age caught up with ol’ Sparky.
The Far Side, which is only made better by every sub-par imitation which gets released.
Calvin and Hobbes, with Frazz a fairly good homage which needs to take more chances.
Get Fuzzy, which has made me laugh harder than any other comic strip.
Pearls Before Swine, which is not far behind.
Little Nemo In Slumberland for the sheer artwork and imagination and just being Winsor McCay at his Winsor McCay-i-est (and that ain’t peanuts).
Krazy Kat for being jazz in comic form, repetition on a theme done with wit and aplomb and endless language play.
Not Worst, or knockwurst, the sausage of the comics world, a homogeneous product laid out on the page, endlessly, repetitively:
Any of the zombie comics, such as Blondie or Hi and Lois or Beetle Bailey or The Lockhorns or any of the other comics passed down from generation to generation, the ones which endlessly rerun the same jokes which may have been funny once but which have now been so thoroughly beaten into the ground it’s impossible to imagine anyone finding them funny. But they’re not the worst. They’re the visual equivalent of cinder blocks, filling space and preventing any holes which angry letters might otherwise pour through. They’re just not worth the effort of considering them the worst.
Worst:
Yeah, probably Mallard Fillmore for being so insufferably unfunny and shrill and chip-on-its-shoulder-y that it’s impossible to imagine anyone laughing along with it in a good-natured way. It’s the comic of the trolls of the political world, the people who only find humor in mean-spirited put-downs and deliberately misunderstanding everyone else’s point simply to annoy them. Sure, there are probably objectively worse comics which have hit print, comics with even worse art and, possibly, meaner “humor” and a lower average intelligence, but nobody’s heard of them so their damage is limited. Mallard Fillmore, the Affirmative Action comic, the comic which only gets printed to “balance out” Doonesbury, is running in wide release.