I like the Top of the World restaurant at the top of the Stratosphere. It's a beautiful view of Vegas, and it rotates (I think 360 degrees in one hour). The food is very good too.
Mandalay Bay has lots of beautiful restaurants. I've only eaten at the Cafe though. (We went to see Mamma Mia, which was excellent!)
If you're willing to go outside of Las Vegas, trying looking at restaurants at Lake Las Vegas (just outside of Henderson). There's a Hyatt Hotel that's supposed to have a really good restaurant, and there are restaurants at the Casino Monte Lago and the shops at Monte Lago. The landscaping is gorgeous out there (Celine Dion and other stars live in the gated community there), and it's on a man-made lake. There's also a Ritz Carlton, but I haven't heard anything about the restaurants there.
I've heard that Bellagio has a great buffet too, if that's an option.