Re: What is the hardest Slam to win?
Wimbledon, I'd say. On the women's side since 1990, look at the Wimbledon champions.
Steffi Graf - 5
Venus Williams - 4
Serena Williams - 2
Martina Navratilova, Conchita Martinez, Amelie Mauresmo, Maria Sharapova, Jana Novotna, Lindsay Davenport, Martina Hingis - 1
I didn't even count the two previous Wimbledon slams to 1990, both of which Graf won. Seven slams NOT won by a sister or Graf out of 18. It's even more lopsided on the men's side (Sampras - 7; Federer - 5; Edberg, Stich, Agassi, Krajicek, Ivanisevic, Hewitt - 1).
You pretty much have to be the best to win Wimbledon--the only "surprise" Wimbledon champion since 1990 is Conchita, and even then, you just thought she'd get the French Open first, she had game on grass.
But all of the slams are very difficult to win, that is where I find them equal, even if not in prestige. Props to all slam champions.