We love the Studio B Buffet at the M. It is probably hands down the best buffet in Las Vegas.
Pros: The M sends out coupons for the line passes which helps because there is always a line. The food tastes great, the service has always been good. They have free wine and beer plus the dessert station is amazing. Gelato and espressos, lattes, and anything else you can think on.
Cons: There is always a long line so be prepared to wait and it is pretty expensive as far a buffets in Vegas go but worth the money.