There are no formal rankings; don't waste your time looking for that.
Don't get hung up on the "prestige" issue. There are some programs that are larger/older/better funded/more famous/have bigger names than others, but that doesn't necessarily translate to "better" in terms of your interests or quality of training. Anyone you ask will give you a different opinion on what is the "best" program (usually it's the one
they went to
)
Check out various program websites for starters. The keys to a good program, I think, are size and diversity. I'd advise you to look for programs that are large in terms of patient capacity (so you see lots of cases) and have a very well integrated multidisciplinary approach (i.e., have docs who are from pulmonary, neuro, and psych, not just predominantly one area) as well as lots of interaction and rotation time with ENT, maxillofacial surgery, dentistry and psychology.
Another thing to consider is whether you are interested in private practice vs academics. If you want to play the high-power-academic game then yes, it probably would be nice to have a big-name program on you CV. Also, it doesn't hurt to have the connections if you want the program director to drop your name for jobs at other academic institutions. For private practice, this is much less important. Private practices want people who know their stuff and can work quickly and efficiently and are agreeable to work with. Period. They really don't care about your 27 publications and poster sessions.
If you are interested in research, look up publications on the web from people at various programs to see who's doing what and how much it interests you. Sleep fellowships are brief and there's not much time to get projects done, so often your research will involve piggybacking onto an already existing or established line of inquiry. It's helpful to go someplace where they are doing stuff that sounds interesting to you.