The easy answer for me is the Flame Tree... but the best rib I ever had on Disney property was the Kenyan Coffee BBQ-Braised Beef Short Rib at Jiko. It's a single, giant very meaty rib, quite possibly the most tender rib I've ever had.
I had thought -- CS ribs, how good could they be? Well, honestly, they were very good! Better than Whispering Canyon. Had both during the same trip at the beginning of this month.
They were possibly my favorite thing I had during the trip.
Most of the ribs at the WDW restaurants are baked or boiled then sauced and grilled...ok if you like 'em that way, but not real BBQ. Only flame tree does them the right way-- low 'n slow.