Just about every hotel on the Strip has at least one show (usually several) and many of the Downtown hotels have shows as well. With so many options it is hard to decide which to see when visiting. We've done some research and compared a number of ratings and have come up with this list of Vegas must see shows:

"O" at Bellagio - Cirque du Soleil has a lot of amazing shows in Vegas, and around the world, and you're going to see many of them on this list. "O" is an incredible aquatic performance, the only one of its kind. It is perhaps the most unique and popular show in Las Vegas.

Le Reve at Wynn - An absolutely fantastical show exploring the subconscious. Amazing performers take advantage of the custom stage, entering both from the sky and from the 1 million gallon tank below.
Phantom at Venetian - A live musical performance after the classic play Phantom of the Opera, but with a more modern feel. Don't let the "musical" part scare you away (if you happen to not be into musicals) Phantom is really cool.
Zumanity at New York-New York - Another amazing Cirque production. This one is based on the human body. You'll find Zumanity on most top ten lists, we put it a bit higher because it's all about sexuality and Vegas is a sexy city. There is nudity.

LOVE at Mirage - If you like the Beatles this may be you first choice. If you love the Beatles LOVE is a reason in itself to make a trip to Vegas, and even if you don't particularly care for the Beatles this is still an amazing show. Another Cirque production.
Crazy Horse Paris at MGM Grand - Based on the Crazy Horse Cabaret in Paris, Crazy Horse Paris is a celebration of the female body with talented Parisian dancers, ballet artists, and is full of singing, dancing, and comedy. This show is a lot of fun. There is nudity.
Mystere at Treasure Island - Another beautiful Cirque show. Great production and the performers are amazing. A highly athletic and high energy show with amazing costumes and great imagery.
Blue Man Group at Venetian - If you are in the mood for something different than this is it. A vastly imaginative, interesting, and fun experience. Blue Man Group has been a staple of Las Vegas entertainment for years and is now popular in Europe as well.
SINATRA Dance With Me at Wynn - Sinatra has become practically synonymous with Las Vegas, naturally there would be a show dedicated to his great music. SINATRA Dance With Me features favorite songs and some of the most talented dancers in Vegas.
KA at MGM Grand - A high flying story of love and adventure by Cirque. This amazing performance weaves a beautiful story over a number of landscapes and is in the top three Vegas shows on several lists. If you like a strong story with your performance art, this may be your top choice.