While I love USO and IOA, MK is by far the best theme part. More rides, more shows, move diverse experiences and its the one park that is virtually impossible to do everything in one day. I will say IOA and USO are getting harder to everything in one day as well. I would rank IOA second and USO third (now that Potter is there). DHS, EPCOT and Animal Kingdom are just riding the coat tales of MK.
Have to admit I have not been to Discovery Cove, but have to make that a side trip soon.