50s Prime Time Cafe is the quintessential Disney restaurant experience. I wouldn't say it is the best restaurant or has the best food of all of Disney's restaurants, and it isn't my "favorite," but I don't think you can say you've experience Disney dining at its best until you've eated at Prime Time. Prime Time is what Disney dining is all about: Putting on a show, and at the same time putting you, the guest, into the show.
The Rainforest Cafe in Burlington MA is far superior to the two Rainforest Cafes at WDW, so I couldn't imagine ever wasting my time at a RFC at WDW ever again.