I always stay at the Regal Airport for my overnights at HKG. You just can't beat the convenience of walking to/from the terminal in 3 minutes. Also, you have immediate, 24/7 access to all the terminal concessions, AirportExpress, local buses, taxis, etc.
Regal also has a good gym and two pools. Good WiFi. Rooms are not super luxurious, but just fine...you're not going to spend much time in the room itself anyway. Restaurants on site are supposedly good, but expensive. I've never tried the restaurants, since the airport is right next door and has a bigger selection (and quick access to Tung Chung via bus/taxi, as well as short Airport Express ride to Kowloon or Central).
Marriott Skycity is actually walkable to/from the terminal too, though outdoors, if you'd rather not take a shuttle bus. Takes about 15 minutes or so, over a pedestrian bridge from the other terminal (shopping mall/IMAX theater) and then walking through a couple of parking lots and crossing a road. I might stay there if I had a longer stay and did not want to stay in town for whatever reason.