The Bellagio has a buffet that is pretty spectacular, as far as buffets go, BUT, it is more of a showcase of individual stations, where things are prepared to order, right then and there. A wide variety of regions, the quality is actually good, unlike most buffets, and the pastry department is worth the visit alone. Mind you, it's like $40 a head, but that isn't bad for the amazing spectacle that it is. A generous slice of actual PRIME rib(with sides), sliced to order, some beautiful Maki Rolls, made to order, and an amazing dessert for $40 isn't bad.
The Rio World Buffet I would give equal accolades to. Not the typical "Number 1 Buffet Chinatown Express" type place on thinks, when they think buffets.