Very tough indeed, but I'm going with Mr. Woods.
It's Tiger, easy.
Not only has Tiger won ALL the 4 slams in 12 months for a "Tiger Slam", he was won every single grand slam at least twice.
Federer hasn't even come close to wining the French Open.
Lemme know when Federer actually wins all 4 slams, let alone win them in 12 months.