No matter how good the ride, 12 hours would be a nightmare!! That said, love RnRC, and I started to say I would hate to be on It's a Small World, but now am second-guessing that. Might be able to curl in a corner and sleep a few hours away...
Best-TTA, Haunted Mansion, Splash Mountain, Spaceship Earth or The Living Seas.
Worst-ToT(even though it's my favorite ride!), Mission Space, Space Mountain, BTM, or Dinosaur.
I would ride Soarin' all day. So far I think the max I've been able to do is 14 before the lines and lack of Fastpasses made me stop.
I once rode the Batman roller coster at Six Flags Great America 30 times in a row. Only reason I stopped was that the park closed. It was at a private party type thing, so they weren't even making you get off except to change seats if someone was waiting for your row.