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Wayne Gretzky. In the history of the NHL, no one even comes close to him. He was the top scorer 10 times and the best player 9 times. He performed miracles for the national team. Even in confrontations with the "Red Machine," his record in the Canada Cups is 2:1 (the 1991 tournament doesn't count, as our team brought a toy car that couldn't even leave the pit lane). I consider the final of the 1987 Canada Cup to be the best event in hockey history. Three incredibly intense matches, played at an unbelievable level, ended with a score of 6:5, with all the truly strongest players playing for each team. Suffice it to say that this was the only tournament where Gretzky and Mario Lemieux played for the Canadian national team at the same time.
In general, that's how it was. Although Gretzky once said that Russians shouldn't root for me, as Russia has plenty of its own Gretzkys, but that doesn't diminish his greatness.
Regarding Schumacher... Well, he won the championship 5 times in a row when he clearly had the best car compared to his rivals, and the rivals themselves were not as strong. Therefore, I don't consider him better than Prost and Senna, who won the same 7 titles between them, competing against each other, plus they had Piquet, Mansell, and others challenging them.
Tennis players... Until someone wins a Grand Slam, it's inappropriate to talk about the best in history.