I've stayed at Boardwalk, Dolphin, and Yacht Club. I've also stayed at CB, but don't really consider that an Epcot resort.
Yacht Club was recently refurbed when we went and we loved the room. Amazing pool. My only complaint was that the walk to and from HS is longer than Boardwalk.
Dolphin - I'm not sure if we got a crappy room or what. Prices were good, and the bed was nice, but the room itself was very cramped compared to the other resorts. The bathroom was TINY, small enough that it was difficult to enter the bathroom and actually close the door. Additionally, we were on the 2nd floor but had to use the elevator to leave the building. We prefer to use the stairs whenever possible, and that's relatively easy in the other resorts in the area. The pool was very nice.
Boardwalk is home. Love the resort and the atmosphere, it truly feels like a day at the shore. It has about the most even walks to the parks. Love seeing the performers on the boardwalk when we walk back from Epcot. It has the weakest pool of the Crescent Lake resorts.
Caribbean - it's not in the same category as far as theme and quality. The rooms entrances are exterior access. Haven't in several years so I'm not sure how nice Old Port Royale is. It will soon have Skyliner access to the parks and it is certainly cheaper than the Crescent Lake resorts, but I'd still skip it.
I'd say Boardwalk or Yacht.