Obviously voted for Calvin, but I won't ever say Garfield is the worst comic strip. I will take a Garfield comic any day of the week over something like Hi and Lois or Blondie.
What's infuriating about Garfield is it's almost okay. The comics have the cadence of a good joke, but recycle the same punchlines over and over. Garfield has the potential to be great - to go for an unexpected twist in their jokes, to turn into wacky cartoon slapstick, or to go into actual storylines like that Halloween comic. But it never does. Garfield always plays it safe. Garfield earned its forever spot on the newspaper page, it clearly has no intention of rocking the boat anytime soon.
Calvin of course being the antithesis to all that. Waterson really set out to have an unpredictable comic strip with story and heart, and at the end of the day, he lost that battle with a syndicate that wanted anything but that on their page. I wanted to be a newspaper cartoonist around that time in my life, and Waterson and Larson retiring almost back to back was pretty seminal to me - newspapers weren't the way to go.
Sidebar? Real talk? The animated Garfield and Friends show? Supremely underrated. The secret best animated show if its time.
It uses the comic strip as its base and actually goes into meta humor of all things - the writers are self-referential, pointing out animation mistakes, voice actors having multiple roles on the same show, commenting on the commercialization of other 80s cartoons (Garfield included.) It's right up there with Animaniacs for smartly-written cartoons, and it never gets credit because, ya know, it's related to Garfield. The bad unfunny thing.