This is the reason we stayed at the MGM. Ok, the price for our rooms also helped but the pool at MGM is by far the best on the strip. I love that there are five different pools to choose from and the lazy river is amazing. The only downside is that there are very few places to get some shade while you're at any of the pools. If you want an umbrella either get there at 8AM and stake your claim on the five that are there or pay for a cabana.
Since the lazy river seems to be the most popular place at the pool complex, a quick tip. Get in line at 9AM with a pre-determined location of where you want to sit all day. The lazy river opens at 10 and if you want shade (under the bridges or near the walls of the bridges) you should get there as close to opening as possible. It's a mad dash to those spots when the gates open at 10; almost feels like Black Friday.