The Bellagio buffet gets my vote as the best buffet on the strip. I cannot count the amount of times I have been here, but every time, it is excellent. I do not really recommend the breakfast or brunch, as much as I can recommend the weekend dinner. While the breakfast or brunch is very tasty (especially the nutella and banana stuffed french toast), it is pretty standard fare. The weekend dinners typically include the king crab legs, jumbo shrimp cocktail, kobe beef, beef wellington, and rack of lamb. All excellent entrees. The swordfish and mashed sweet potatoes are also excellent. Come early (around 5ish) and with a good appetite and you will not be disappointed.
Also, the desserts are high end restaurant quality in both taste and presentation. You cannot go wrong with anything you grab from that station.