I can't believe no one has mentioned Liberty Tree Tavern yet! My family indulged in many of the most popular dessert restaurants on our trip (dessert is the highlight of our meals at WDW), so I think I can add quite a bit about this one. We've had creme brulees practically everywhere (except Downtown Disney), the best ones were probably the maple creme brulee at Le Cellier, the key lime creme brulee at CRT, and the roasted strawberry creme brulee at Brown Derby. We've had the bread pudding at 'Ohana (ok, but not great in my opinion), the Cinderella's slipper dessert, the mini wedding cake and castle cake (both also ok but not great - they look better than they taste).
The best overall restaurants for dessert on our trip in September were Coral Reef, Tony's Town Square, Kona Cafe, and Liberty Tree Tavern. Liberty Tree Tavern won hands down for taste, especially with their homemade (or homemade tasting) ice creams that accompanied each amazing dessert. The chocolate lava cake with cherry ice cream was wonderful as was the butter griddled pound cake with pecan caramel sauce. And although I didn't get the chance to try it, my sister ordered the apple tart which turned out to be a small-medium sized pie with shortbread crust and cinnamon ice cream, and it was her favorite dessert of the entire trip. It looked amazing and it was my sister's favorite dessert of the whole trip. Kona Cafe wins for presentation. I ordered the new citrus mousse dessert there last time and it was very good. The new Celebration citrus trifle dessert at Rose & Crown was very good too. Oh, and I'll second the Dole Whips, especially the Dole Whip Floats - they're very refreshing and we can't leave WDW without getting one.