Probably either Jack Kramer, Pancho Segura, Rene Lacoste or Bill Tilden. Segura was a brilliant player and teacher of many great players like Jimmy Connors. He advised players like Hoad and Rosewall. Lacoste kept very extensive notes on his opponents, invented the ball machine to help him improve etc. However I would go with Jack Kramer. It's very close between all four.
Here's a little about Jack Kramer from Vic Braden's book Mental Tennis--As an analyst, he was unparalleled. He could watch a player once and tell you exactly how to beat him. I talked to countless players who told me that Jack had zeroed in on exactly the flaws that guaranteed he would beat them.
I'll give a story about the genius of Tilden from the great Fred Perry. This is from Fred Perry-An Autobiography. Many years later, having played each other about 300 times, we happened to be in Independence, Kansas. We weren't due to perform until that night, which was just as well because the heat was fierce. Tilden called my hotel room and asked what my plans for the day were. I told him I was going to sit under a tree, relax, and smoke my pipe. "I want to go out and hit a few balls with you, " he said. I told him, "For crying out loud, Bill, we've been playing daily for two months and we've go a match this evening. You want to go out in the heat and hit? You must be crazy." He insisted, however, saying, "I want to show you something."
When we got to the court he asked me to hit a few to his forehand, low and wide. I did this and he returned them using a perfect continental grip, just as if he were mimicking my own forehand. When I inquired what he was up to Tilden said, "After playing so many matches against you and studying your style, I realized that the continental grip, and not my own Eastern grip, is the only one for that sort of shot. I felt I wouldn't be the complete tennis player unless I had mastered it to the stage where I could use it in a match if I wanted to."
When Tilden perfected that continental grip, after the war, he was fifty-three years old.
Here's most a little later in the book-Another ruse of mine, when I went to the net, was to hit the ball and then move my body behind it, making it a little more difficult for my opponent to pick up the white ball against the white background of my shirt, and maybe upsetting his timing slightly. The only player who noticed this was Bill Tilden. He told me in Omaha during one of our summer tours that he had been bothered with his eyesight lately, particularly playing indoors, as the ball "sometimes seemed a bit hazy." He had just visited the optician, but had mysteriously been found to have perfect vision. And then, looking me straight in the eyeball, he said, "You know something Fred? When you come into the net you move in behind the ball so I don't get a clear picture of what I'm aiming at" Tilden never missed a trick.