Depends what you're looking for. Solely for the reason of inhibiting aromatization it's undoubtedly letrozole aka femara.(either that or aromasin, haven't read much on it)
However, if you think there's a chance that part of your problem is elevated IGF-1 levels then you wouldn't want to use letrozole because it elevates IGF-1 by 24%. This can also be a good thing though.