In the open era, I will list Lendl's forehand as the best ever over Federer and Sampras.
Lendl has a more consistent forehand than Pete and is at least as powerful as Pete's.
Compared with Federer, I think Lendl hits it with more power and still not losing in the consistency department.
Federer at times can be overpowered by other big guns even in the forehand side. For example overpowered by Del po in this US Open. Lendl however is rarely overpowered in the forehand rallies.