This is a toughie. Sitting down with a cafe au lait and a pain au chocolat (chocolate croissaint) to watch impressions of France in EPCOT is a great food moment for me.
My very first Guiness was at the Rose & Crown -- I was about 14 at the time! That's a special moment.
But really, the best single thing I've eaten at WDW.... I'd go with the smoked salmon tartar at Bistro de Paris back when I still ate seafood. That was part of the best single meal I've had at WDW -- and one of the best of my life (so far).