At it's best, Federer's forehand was an irresistible force and the most potent forehand I have ever seen. It is quite clear to me that it is substantially superior to the Sampras forehand. As for Lendl, I simply haven't seen enough of him but to be sure his forehand looks powerful, consistent and also versatile, though even from the little I have seen it does not look to be the potent whirlwind that Federer's was/is. As for Segura, I can only go by what I have read really, in that his shot has been considered by some to be the best shot in the history of the sport.
It is pretty hard to argue with Federer winning 15 slams (and counting perhaps) with the forehand being his biggest and best weapon. The discrepancy between him and Lendl to me seems too great to place Lendl above Federer in this list. To be sure on that inference I really do need to see a bit more of Lendl's forehand though.
"If you're gonna be 5'8'', you better not be bald, fat and drunk whilst asking to be put in the 'all-time great sportsman' conversation." - Zoid