If you're a local, Station Casino's boarding pass is obviously the best. They give you stuff you can actually use, including such wonderful items as cordless drills and glass pasta containers. For tourists who are looking to parlay their losses into room offers, Harrah's Total Rewards program is pretty generous with throwing rooms at you.
The only problem with Total Rewards is that they comp rooms at Harrah's joints only. If one could use the same comps for rooms at other hotels, but give a three fingered promise to play at Paris Bill's Faballysulous Imperial Flamingo they'd really be onto something there. If you are going to use Total Rewards comps, request Flamingo Go Rooms or Caesars Palace. Just don't expect to have any luck whatsoever on their machines - at all.