Nadal can hit both flat and with superspin when at his best. Olderer can hit flat, but his pure topspin FH is significantly weaker, there is a reason why Nadal has the greatest FH down the line and passing shots. The thing that put Olderer's FH above Nadal a few years ago is the pure flat drive, which Nadal somewhat lacked. After 2008, IMO, Nadal's FH has become the greatest shot in this sport.
Not sure when you started following tennis, but your comment about forehands down the line is completely incorrect.
When Federer was in his prime, his down the line forehand was arguably his main strength. He would often finish rallies which appeared to be in a neutral position by cracking forehands down the line and following into the net anticipating the retrieved slice. The difference with Nadal being that he doesn't have the ability to hit flat. Federer didn't need (and still doesn't when he's confident) to loop the ball over the high part of the net, he had a knack for precise timing and feel that allowed him to choose the more aggressive option.
Nadal's crosscourt forehand is the far more effective shot simply because he's left handed and can generate so much spin to create angles to open up the court. However, this play can be neutralised by not allowing Nadal time and hitting flat and hard. There's not much you could do about Federer's forehand because he once thrived on pace and if you gave him time, he'd happily put the ball wherever he wanted.