You have lots of choice very close to the terminal. It really depends on your budget and whether you plan to spend any time at the hotel or just a minimal layover. Hyatt Regency, ITC Maratha, Leela Mumbai are all v. nice but pricey and the facilities (pools, gym, spa, fine dining etc) may be wasted if you're only going to be there a few hours. I usually get a club floor room at the Leela - the club is a nice place to relax and the package includes 2-way transfers. For a 5.55 am departure I would aim to reach the airport at 4.30, therefore leaving hotel at 4.15.