This has to be categorized by era. The modern racket since the late 90's has changed the fh very much. Nadal's heavy top spin fh for example can't really be compared to players from the 80's because they can't be done in such way (or vice versa).
Federer for sure will top my list on the best forehand.
On Power (hard court) I will pick Safin, Gonzo, and Delpo.
Rafa is up there but I have some reserve since his lost to Soderling in RG. But it could be just an off day...