Hi Agassiman,
The real answer is, Lendl, but it's quite close. Sampras' backhand is highly underrated, by...um...the ignorant. LOL. It held up well under persistent attack from people like Agassi and Courier. Lendl's actually, often resorted to chip underspin when under attack from players like Agassi. Lendl's chip, was good enough to prevent consistent attack, but it was hardly overwhelming, just enough to keep the point going and going and going (which Lendl was very happy with).
Sampras' actually had a better topspin high roller, than Lendl, due to his grip which was further around than Lendl's continental. It also made it easier for Sampras to take high bouncing balls, (lendl often had to slice them, though he did have the upper body strength to just muscle them when he had to).
People often forget that Lendl didn't really blow people away with the backhand all that much. He mostly used it to stay in the point (slice), move the ball around (his topspin drives), and set up the forehand or get the error. His biggest blasts were often saved for the passing shots. Here, he could be deadly, especially after he developed a great crosscourt pass(a weakness for him early on). But, Sampras was also a sharpshooter on passing shots.
Anyways, overall, Lendl, but it's not nearly as one-sided as people here portray.
