My grandmother who is a Disney fan and in a wheel chair would vote for EPCOT as well! I can only think of Malestrom, Test Track, and Mission Space being a problem.
I would give a very close second place to DHS just because of the shows and basically there are only three attractions that might give a wheelchair inhabitant pause.
Universal Studios Florida, Disney's Hollywood Studios, and SeaWorld must be up there, too, due to their abundance of shows.
If you can transfer, then you shouldn't have much trouble getting on any ride. On almost all rides they allow you to stay in your chair through the queue all the way up to the ride itself, then you transfer.
If you can not transfer, then you will be limited to which rides you can take your wheelchair on. As the others have stated, Epcot allows the most rides with your chair, off hand I don't remember them.
Have you checked out Disney's map for disabled people online yet? At guest services, you can get an actual map with all the rides listed showing the symbols if you need to transfer or are able to take your chair on the ride.
Is your wheelchair motorized? If not, then you may need someone to assist you, like Zachary helps me. If you have strong arms, you shouldn't have much trouble getting around. Magic Kingdom does have a few hills but Animal Kingdom has a long hill to get out of the park. Epcot and Hollywood Studios are okay.
Hope this helps you Daniel, if you have a question, just post it and I'll see if I can give you an answer. Oh yeah, I just remembered one other long hill, the one to get into Fantasmic at Hollywood Stodios.
Walt Disney World
Tokyo Disney Resort