After the Wynn, I think the Bellagio is the best dinner buffet in Vegas. Clean, big, interesting choices, nice room. They have things like salmon and tuna poke (a sushi variety) and incredible meat and fish items. I always try to make it to the Bellagio buffet when I'm in town. A bit pricey but better than a plated dinner and if you skip the optional cocktails you can get a HUGE dinner for two with top quality food (crab legs, sushi, prime rib, beef, pasta, killer desserts) for under $90 with tip.
I'd rate it an A.