It's so hard to pick just one, even for each park. I'll try my best:
(This is what rides I assume I'd just ride over and over all day.)
Over all my all time favorite ride is Spaceship Earth.
Magic Kingdom: Pirates of the Caribbean, Haunted Mansion & TTA
Epcot: Spaceship Earth & Maelstrom (just the ride, the movie doesn't really tickle my pickle)
Disney Studios: I honostly can't think of a ride that I would sit on all day (I haven't been on the Backlot Tour for like 10 years though, so it could be that)
Animal Kingdom: Primeval Whirl, Kali River Rapids & Kilimanjaro Safaris.