For the younger set, generally The Palms or Hard Rock Cafe or Mandalay Bay (my personal favorite) are the "hot" hotels--but there are tons of good ones there, so it depends upon what you are looking for.
The Palms is known for its nightlife, Hard Rock Cafe for its pool and "big-name" rock stars that perform there and Mandalay Bay is known for its "wave" pool--and for its many cabanas as well as its European-style pool (which I've actually never visited).
All are good choices--and if you are looking for good places on even more of a budget, then consider Imperial Palace or Harrah's or even Venetian or Encore as mentioned above.
For "luxury" consider Caesars Palace. Bellagio and MGM Grand are also good, but Bellagio is kind of empty lately and MGM Grand is just so spread out and the pool is pretty small. Mirage is also quite good with a great pool and some interesting bars and shows--and Treasure Island next door is a "sister" establishment.
Enjoy