I lived in Las Vegas for two years for graduate school and although I've found that locals typically don't eat at buffets (lol, probably because it's pricey), I've been to the following casino buffets at least once and they've consistently been very good. I have put the meals I've eaten there beside each casino's name.
1) Mandalay Bay (BREAKFAST): This is my absolute favorite buffet in Las Vegas to go to for breakfast, I haven't found another buffet that even comes close. The variety of food is very good, there's something for everyone from traditional to regional and ethnic breakfast items and it's all very fresh. Breakfast averages around $17/pp.
2) Rio (DINNER): If you don't mind crowds (and it can be very crowded, come early as the line gets ridiculously long), the Carnival World Buffet offers cuisines from South America, Mexico, China, Japan and Italy. This place serves just about everything from sushi and edamame to fajitas and fried rice. Average price is $24/pp.
3) Paris (LUNCH): Le Village Buffet offers regional French fare. You can enjoy a taste of a traditional French bistro and the stations are dedicated to different French provinces. Average price is $25/pp.
4) Bellagio (DINNER): Professional chefs make food in live cooking stations and the variety of food is very good at this buffet (try the venison and Kobe beef!). You can also request Italian, Japanese and Chinese items at this buffet and they also serve standard American fare. Average price is $32/pp.
5) Planet Hollywood (CHAMPAGNE BRUNCH): The Spice Market buffet restaurant offers an assortment of cuisines like Mexican, Italian, Middle Eastern, American and Asian. Chefs prepare and serve food at designated booths, and the unlimited champagne brunch is a huge hit (very crowded). Average price is $23/pp.
Remember that you are going to get what you pay for, so be prepared to pay a little more for better quality food.

Have a great holiday! Oh, and a great way to save money on the buffets is to join the players club at the casinos, often they'll include a few buffet when you join.