Best:
The Simpsons - customized chalkboard & couch scenes... I mean, c'mon! An original idea (almost) every time! Oh, and a great score by Danny Elfman.
Futurama - now with more Slurm!
Family Guy - F'n great
King of the Hill - Great "hillbilly" style music, introducing all the characters, etc.
Justice League - Computer Animated superhero's and a great symphonic score!
Batman: The Animated Series - what everyone else said. A fluidly animated title sequence of Batman whoopin' ass. They don't even NEED to print the words "Batman" onscreen for the viewer to know what they were watching.
Enterprise - An inspiring montage of human achievement, and a glimpse at what we can achieve as a species if we learn to learn & accept each other... as a species (and I love the music too).
Star Trek TNG/DS9/Voyager - Great CG/Model work of the great void of space, backed up by a creatively produced symphonic score.
Desperate Housewives - Very Andy Warhol'esque with a cute soundtrack to match.
Worst:
Any reality show - Ooooo Dramatic! Oh wait, maybe not.
Justice League: Unlimited - They took a great opening montage with a symphonic score, and turned it into clips from the CURRENT episode (thereby ruining it for the viewer), with a riffy rock-guitar soundtrack. Blech!
Teen Titans - Oh man, is this ever ANNOYING music! Sounds like the Chipmunks singing, AFTER they sucked down a few helium filled baloons. UGH!
No more come to mind at the moment, but I KNOW there are several that I despise.
Moe.
