Fischer.
1. When he got a positionally advantageous position, he never seemed to make a technical mistake and would grind out a win like a computer.
2. His powers of concentration and coolness during play was second to none, which enabled him to cope with surprises during the openings.
3. He relied only on himself in chess. No vast organization behind him with an army of GMs to help him in openings, novelties, etc; he never really used seconds; had an amazing will to win. He played every game as if it was the last thing he was going to do on earth.
Kasparov's concentration has been known to lapse and he often lost motivation and the will to defend in tough situations (as seen in games where he was surprised in the openings). He had the state resources and GMs to help hi - and later computers. Fischer just worked everything out in his head.
Put these two on a desert island with only one chessboard to use when playing each other, and Fischer's natural technical ability would easily see him get the upper hand.
Ok Let's go
https://en.wikipedia.org/wiki/Comparison_of_top_chess_players_throughout_history