It's hard to say there's a best hotel in Las Vegas but when you consider the entire package - service, location, rooms, facilities, et al - the Mandarin is at the top of the list.
There's no doubt the Encore, the Wynn, the Four Seasons, Bellagio Villas, and MGM Grand Mansion are competitive but they all lack the service and style the Mandarin delivers. As a non-gaming hotel the Mandarin offers a zen like retreat with ready access to the strip.
The other reviews will give the details. However, as someone who has hit all the high spots in Vegas, I post this to say I keep coming back to the Mandarin because it is the best. More