Attempting to put a value on Federer and Nadal's forehands without considering the degree to which their unmatched footwork contributes is a task bound for failure.
yes you have to incorporate some footwork anyway, that's why I may think Fed's forehand peak is better than Nadal's, I'm not sure about that but he had such a phenomenal combination of footwork+forehand at his peak, but at the moment Nadal's is clearly better than Fed's imo.
I would say :
1. Nadal (not as much variety as Federer but so violent and reliable)
2. Federer (Luthi said that it's the barometer of his game, its quality varies more over time than his backhand's contrary to what many people think because his backhand is his weak side ; besides, this shot has a lot of variety and has evolved a lot across the years, which is surprisingly very seldom talked about comparing to the whole fuss which has been made to justify that Nadal managed to win slams on other surfaces than clay)
3. Del Potro (overrated by some imo : it's a great solid powerful shot but not "consistantly lethal" as some describe it, if it was he would make more winners ; Gonzo's forehand was more effective imo and I would put Gonzo's forehand in competition with Fed's and Nadal's, I would not put Del Po's there)
4. Tsonga (it's not only about power, he can also use spin quite well now, his footwork is the problem : we come back to the difficulty of separating this shot and the footwork although we can try this imo)
5. Djokovic / Ferrer : Ferrer has a better control on this shot but Djokovic has more power, surely all the people who are obsessed by power on this site will say that Ferrer's is much worse than Djoko's and Berdych's
Ferrer's angles and direction changes are easily worth Djokovic's )
7. Berdych : it's not as good as his backhand, he can hardly play it with spin which is a very dangerous game : he deals well with it but it's far too little reliable comparing to Ferrer's and Djokovic's
8. Murray : now I think it's better than Monaco's