I'll preface with the fact that my father is a retired physician and I am a medical school student. Also let me add that we haven't been to every hospital in Vegas, but we've been to damn near all of them. With all that said i'll tell you right now my father will only admit himself to two, Summerlin, and St. Rose.
I'd say hands down that Summerlin is the best hospital in the city, for grownups at least. It certainly is probably the prettiest hospital. The staff are courteous and nice, and the triage is pretty good at prioritizing.
Also the 6th floor rehab wing is amazing. The PT's OT's and Dr.'s really help make sure patients are good to go home.