I hate King of the Hill so much. I never thought it was funny...
The old episodes were actually really bad, basically because the writers and artists were still in their Beevis and Butthead stage and didn't know what to do with the characters, plus the writing was just really bad.
After the show developed it got hella better, especially the last few seasons.
I'll admit, it still requires a certain humor to enjoy it since the humor is pretty dry or implied, and not in-your-face-here's-where-the-humor-is of Family Guy, etc.
My favorite shows that are still on atm:
Glee - /me too Gleek
Futurama - still one of the best, if not the best, animated shows ever created and the humor is just amazing. The story is also great and allows for a lot of show variance, which is always nice. I've definitely seen every episode at least 20 times, mostly because Adult Swim played through the seasons so much after it got canceled, and then Comedy Central picked it back up again.
NCIS - Yeah I know a cop drama, but NCIS is just so much better than most of the other cop ones, especially Law and Order and CSI; the lower amount of cast and the length they've been on the show really gets you attached. Pauly Perrette and Mark Harmon are just amazing in this show, with Abby probably being one of my favorite characters on TV ever. It's also easy to get into the show, you could start watching episodes on TV right now and figure out what's going on even though the story is constantly progressing.
Spongebob and Adventure Time - Best. Cartoons. Ever.
EDIT: Big Bang Theory - absolutely amazing writing and cast, definitely one of the shows you need to watch at least once.