I am definitely a fan of the Mandarin Oriental (Bar)!
It's beautiful with sweeping views of the strip, intimate bar area, live music, and exotic eats. It's definitely on the pricey side, so get ready to spend some dough, but it's worth it! The bar is tucked away on the 23rd floor of the hotel, to the right of the registration/lobby area. It boasts floor-to-ceiling windows and seats all around with great views. The menu has some great drinks and mocktails (I had the refreshing watermelon/cucumber/mint) and small plates that are innovative and delicious! Order the cool BBQ for the spooky dry-ice effects and order the Baked Brie for 2 because it's yummy!
Service is solid. Our waitress made a mistake on our order and immediately took it back and was very sweet and apologetic. We never felt rushed to leave even as the bar started to become standing-room only.
Only complaint is that parking at the hotel is $30, but you can get a parking validation from the bar if you spend....$50+!! (Which for 2 people is easy to do here). Next time I'll valet at the Aria and walk over ;)