The Bayside Buffet at Mandalay Bay is one of the more under-rated buffets in Vegas.
It doesn't boast the largest selection or the most unique creations, but whatever they do have to offer, they do very very well. The food is usually very well prepared and fresh.
My wife and I traditionally like to go there for breakfast for they usually have a pretty good selection of breakfast favorites. Plus, a pretty decent selection of sweets and pastries to pick from if you're third trip through the main buffet wasn't good enough.
We have been there several times and have yet to leave disappointed.