I'm not sure you can automatically get access to a hotel's pool based on your admission to the spa but my favorites, in this order, are:
Qua at Ceasars -- Varied indoor pools/baths (hot, warm and cool), steam room, sauna, and cool/snow room (forgot what it's called but it is a wonderful compliment to the steam and sauna), the size of the place while still "feeling" warm, the variety of teas served in the waiting room/relaxation area, the decor, the fact that I can use my CET reward points there, as a Diamond member fitness center is free and spa I believe is $25 rather than $45 and also with 15% off services. Also, I believe there is more fruit, juices, etc. than at other spas so I've made a mini-breakfast there after working out and before the spa itself. Remember if you purchase a treatment access to most spas is included. I believe Qua also has a co-ed/couples treatment room if you and your other half want to enjoy a treatment at the same time. Some other spas like Canyon Ranch may also have one.
Canyon Ranch at Venetian/Palazzo -- Has a lot of the features above and a larger workout area PLUS there are some neat classes (check the schedule online) that are available for free and that are included in the daily pass. I don't remember there being a snowy/cold room. I rate Canyon Ranch second because though spectacular it somehow felt more cold and less elegant to me than Qua.
Spa at Wynn (Encore's is newer and looks nice from the website) -- Smaller and fewer features but elegant. Encore's looks even nicer.
Those are my top three but I've also visited the spas at Red Rock (small but nice with a steam room and sauna) and Green Valley Ranch (same and I like GVR's a bit better I think). From memory both RR and GVR have small outdoor spa/swimming pools that you can access with your spa admission. RR had a special spa pool with lounge chairs that was very relaxing and I believe -- can't remember for sure -- separated from the other/busier pools.
I've also visited the spas at Paris and PH. Okay. Not great memories but fine. Just not in the same league as the ones above. Same with the Palms, which last time I visited also needed major work. I had the best massage in my almost 48 years there, however, and if the same masseuse is there I'd go back to the Palms to get a massage from him.
I have a thing for the Flamingo spa, which needs major renovation, but it is cheaper, free if you are diamond and I believe the ACG has a coupon for a one-day Flamino spa pass. It is small and has a steam room and sauna. I like that the steam room has a eucalyptus fragrance but the spa is getting worn. It's age though is it's charm. It looks to me a few decades old and I think about the famous entertainers who may have used it in the 70s or 80s (it looks at least that old to me). I also had a great massage there. If are looking to enjoy the spa but want to do it very inexpensively I'd use the Flamingo/ACG coupon and check that spa our for free, especially if you decide not to get a treatment.
Finally, though not a spa, I wanted to add that I may enjoy Qua one day but another enjoy the Flamingo for free and take advantage of the good Groupon offer for a massage at the Tuscany. It's not really a spa but the price for a massage or two with the coupon is great and folks have said good things about the massage. I may even splurge and have them come to my hotel room. Never tried that before and apparently it's not much more expensive to do that if you use the Tuscany folks.
