Southwest and Virgin America are my go-tos, but honestly the flight is like an hour and the whole industry is moderately terrible so United is terrible but not that much worse than average.
There's no Singapore Airlines-style airlines in the US.
If you gave access to other airports besides SFO, realize that OAK (Oakland) and SJC (San Jose) have just as many flights to Vegas and are faster airports on general because of lower volume.
I would recommend leaving any luggage you don't need and just taking the essentials to Vegas. It will drastically speed up your experience at tge airports.