We especially enjoy Wolfgang Puck Express in DTD, Captain Cook's at the Poly, Cosmic Ray's in Tomorrowland, Columbia Harbour House in Liberty Square, Flame Tree BBQ in AK, Tangierine Cafe in Morocco Pavilion, La Boulangerie Patisserie in France Pavilion, Sunshine Seasons Food Fair in the Land Pavilion, and the Pepper Market at Coronado Springs (order the Steak Ranchero). Disney does a great job with quick service dining, IMO. They are mostly wonderfully themed and many have some very unique offerings. Most of the ones I mentioned offer something very different from your average theme park food. Cosmic Rays and Flame Tree aren't that unique, but they have HUGE portions that could easily be split for 2 people, or 2 orders for 3 people. There are so many other places we want to try, especially at the resort!
We don't really care too much for any particular CS place in HS, but Starring Rolls has filling sandwiches and delicious cupcakes (all their desserts are great), and Studio Catering Company has a pretty good and filling Buffalo Chicken Sandwich with fries. Those are the best we have found there, anyway.
Oh, sorry, you said pick one! Well, I'd have a hard time picking between Wolfgang Puck and Captain Cook's...I'd probably go with CC, since it's at the Poly!