In my opinion, you got to have good footwork, anticipation, good ball-sense, good tracking of the flight of the ball and determining acute angles in order to have early preparation to hit a good forehand. So if Federer, Sampras & Lendl are of the same age (assume), using the same racket, using the same shoes, playing the same tennis ball, which of these 3 players have better footwork, better anticipation, better ball-sense, better tracking of the flight of the ball and determining acute angles in order to have early preparation to hit a good forehand??? My bet is Federer. Out of the 3 players, Federer is definitely the most efficient, the better athlete, most graceful, most effortless, best in his footwork, anticipation, tracking of the ball, etc. Sampras is a close 2nd in my books and Lendl next in line. Yes, Lendl was awesome and deadly at smacking the forehand consistently ( I saw "live" 80s matches of Lendl). But if someone like Federer can move him around the court with crazy spins, acute angles, powerful inside-out blasts, variety of forehands, making Lendl run...I doubt Lendl can keep up with Federer. Federer has withstood all the hardhitters of today's modern game by his superb footwork & anticipation. To me, he is an even better "Chess" Player than Sampras & Lendl at seeing the opponent's next move, next ball that is going to be hit, direction, pace, angle, spin, placement. Federer just reads the game better than Sampras & Lendl and this skill of his is not affected by time, by modern technology, by rackets, by tennis balls, by NIKE AIR shoes, by better court conditions, etc. This skill is in-born in Federer, it is 2nd nature to him and he just has more natural talent & more gifted in this department. Lendl may look awesome for balls that are near to him. But the discussion is the best overall Forehand. If you don't include the most difficult scenerio to hit a forehand, then it is hard to determine the best forehand. So let us imagine the worst scenerio:
These 3 players are at the extreme left side / AD side of the court, beyond the doubles line. The opponent hits a ball crosscourt, going to land exactly on the line on the deuce court, with acute angle and sliding away. By the time these 3 players attempt to run, the ball may be ankle low and they almost have to dive or lunge at the ball, their feets cannot be set properly and in mid-air, straining to reach for the ball.....
I believe Federer will have that extra lift-off, quicker off the blocks, better anticipation than Lendl or Sampras and probably be able to reach the ball to make a decent attempt to make a passing shot. Lendl plays very mechanically, and please don't tell me he will be able to run and anticipate better than Federer at getting these kind of extremely difficult balls....
No offense to Lendl or Sampras. They were the Forehand Kings of their generations, but the ONE has arrived to take their places.