This has been discussed many times in these forums:
http://www.chess.com/forum/view/chess-players/who-is-the-best-alekhine-or-fischer
http://www.chess.com/forum/view/chess-players/what-is-your-opinion-about-robert-fischer
http://www.chess.com/forum/view/chess-players/bobby-fischer
http://www.chess.com/forum/view/chess-players/bobby-vs-paul
Anyways....
He . . . was the first guy to get over 2800 ELO rating.
Fischer's highest rating ever was 2785. Gary Kasparov was the first guy to have a rating over 2800, and 2 others have done that since (Anand and Kramnik).
He absolutely wipes the floor with the other contenders for the best.
Fischer himself said that Paul Morphy was the best chess player of all time. Fischer said that nobody alive would be able to beat Morphy in a match.
A simple diagram will explain this entire thing.
Fischer>>>>>>>>>Spassky>Petrosian>Botvinnik>Tal, Smyslov and Bronstein, and Euwe, and Keres, and Reshevsky>Alekhine>Capablanca>Lasker>Steinitz>everybody before him.
This is quite silly, really. Tal actually beat Fischer more times than Fischer beat him.
The problem with comparing modern players to players like Capablanca, Lasker, and Steinitz is: Capa, Lasker, and Steinitz didn't have books written by Capa, Lasker, and Steinitz to learn from - they invented new ideas and wrote the books! Later players were able to study these books, learn from their ideas, and improve on them. If we could take any one of these players in their prime, time-travel them into the modern age, and let them study from all the new books written in the last ~100 years, any of them would be as good as any modern GM.
could somebody show me proof where Karpov, Korchnoi and Kasparov have shown themselves to be better than all of these guys Fischer is better than?
As mentioned in the above paragraph, it's very difficult to compare modern players to players 100 years ago. I think that it is generally agreed that Karpov, Kasparov, and Korchnoi are better than players 100 years ago - simply because more and better chess books are available now, and modern players have computers to study from. Give these tools to players 100 years ago, and they'd be just as good as anyone.
No proof? Well than, obviously Fischer is the best!
This is a logical fallacy known as the "false dilemma". http://en.wikipedia.org/wiki/False_dilemma
Simple really. Robert James "Bobby" Ficher is the single greatest chess player ever. He was the youngest Grandmaster ever (until Judit, that is) and was the first guy to get over 2800 ELO rating. He absolutely wipes the floor with the other contenders for the best. And, he's the only champion to have never lost a world championship match!
A simple diagram will explain this entire thing.
Fischer>>>>>>>>>Spassky>Petrosian>Botvinnik>Tal, Smyslov and Bronstein, and Euwe, and Keres, and Reshevsky>Alekhine>Capablanca>Lasker>Steinitz>everybody before him.
Now, could somebody show me proof where Karpov, Korchnoi and Kasparov have shown themselves to be better than all of these guys Fischer is better than? No proof? Well than, obviously Fischer is the best!