Depends on how much you want to spend and what kind of food you like. There are plenty of world class restaurants in the casinos on the Strip, but you will pay for the food, experience, and the name. In addition to the ones already mentioned, if you like sushi I would go to Nobu inside the Hard Rock. It's not cheap, but it's very good.
If you want to eat on the cheap you need to go off strip. You can go to Ellis Island and have prime rib, baked potato, salad, and a beer for $13. They also have a BBQ on Saturday and Sunday and people line up, sometimes for hours, for it. The small, local casinos are where you are going to find the least expensive food. The buffets used to be cheap, but not so much anymore. If you want variety you really can't beat the buffet at the Rio. Nine Fine Irishmen is a pretty cool Irish bar in the New York New York.