Lotus of Siam has been called the best Thai restaurant in the US. I've only eaten there once, but was quite impressed. I don't think it would be considered an upscale place, but the food is excellent. The biggest problem is its location. It's in a very old shopping center with, shall we say, an eclectic mix of tenants (swingers clubs, gay bathouses, restaurants, bars, etc.). It's not really walking distance from the Strip.

I'll second the suggestion of Commander's Palace. The other/original Commander's Palace is in New Orleans and often makes those "Ten Best Restaurants in the World" list. It's excellent New Orleans food without having to go to New Orleans.

clark