I've done all the big buffets (except the Sterling Brunch) and I have come to the conclusion that Bellagio is my favorite. It isn't the best in terms of service and the lines can be painfully long on weekends, but no other buffet in Las Vegas even tries to offer what they do. I do prefer the weekend "gourmet" buffet to the regular, but their brunches are pretty good as well. The gourmet buffet offers dishes such as steak tartare, Kobe style beef, beef wellington, etc. Also, the brunch has fresh juices and some Asian style breakfast items. Nothing cures a hangover quite like boiled rice.
Wynn has a small advantage in terms of quality. Rio Seafood has better service. Mandalay Bay is the cheapest of the high end buffets. However, overall my favorite after several meals is still the Bellagio.