best move

1.Kd4 winns too, but not that quick.

1...Rxf5 is a big assumption, 1...Bc3 (for instance) delays immediate material losses though the passed pawns won't be stopped.

f6 seems to win material after 1...Bc3? EDIT: No it doesn't.

Great puzzle !! Your opponent thinks that he has won a pawn...and the game is over !

Huh, well, it does seem 1...Bc3 2.f6 wins material... What is Black's answer ? Mine would be 2...Bxf6 3.gxf6 Rxf6+ and the game goes on for a bit (though White should win easily).

I misformulated, but my point was that 1...Rxf5 leads to a tactical sequence that immediately wins, but not other answers.

Yeah sorry it does win material, I looked at the same line as you did and just overlooked that white is material down at the start.