I'd agree with the above poster that the most prestigious research fellowships are HHMI-NIH Cloister, NIH CRTP, Doris Duke, and Sarnoff. Perhaps even in that order? Even more than the fellowship prestige, I think that what you accomplish in your year off is what is important. It probably doesn't say much if you spend a year at the most prestigious fellowship (whichever it may be) and don't really accomplish anything. But again, I can't speak from experience since I haven't done any of the programs. Perhaps some alumni of different research fellowships could chime in with their thoughts?