Not to quibble, but actually all the properties you listed are four star properties. I think for a five star experience you would have to look at Wynn Tower Suites or Aria Sky Suites among the properties you listed. If you're truly looking for a five star experience, you may also want to consider Mandarin Oriental, MGM Skylofts, and the Four Seasons (above Mandalay Bay).

If you just want to be at one of the better properties in Vegas and you don't consider location, Wynn/Encore probably have the best rooms and would likely provide the most luxurious experience. Aria and Venetian Palazzo are probably comparable to each other, although V/P have more restaurants and reasonable options to eat. Cosmo is quite nice but difficult to compare to the others. Cosmo's calling card is the balconies. I've stayed about 10 times at Cosmo and really enjoy the property and think the rooms are nice, but without the balconies and the spectacular views it really doesn't compare. Many people mention the club crowd at Cosmo, but once you're in your room that's a non factor. Also, you mentioned liking the older slots, you may not find much of those at newer casinos like Cosmo and Aria. I've stayed at all the casinos you've mentioned and enjoyed them but it really comes down to what type of experience you're looking for.