By far the best in town quality wise is the Sunday brunch at Ballys.

Its a little pricy ($59 last time I checked), but if you go you will understand why.

As far as regular priced buffets.

The M Resort - Simply fantastic for the price
Bellagio for lunch or dinner
Aria
Rio Seafood (I think this is Friday night)

Remember that you get what you pay for so a $5.99 buffet, tastes like a $5.99 buffet.......