I give respect to all three disciplines and don't think one is better than the other when you look at them overall. However, when you look at each of them I think that:
A. Skateboarding requires more precision and more technical skills than snowboarding or surfing. Flipping and twisting a board that's not attached to your feet takes a lot of practice.
B. Snowboarding requires more guts. Anyone can learn to snowboard, but to throw yourself off a fifty foot cliff or a huge table top kicker jump requires some part of your brain to be different than the norm, and now doing that off a cliff or jump is the NORM for decent snowboarders.
C. Surfing requires strength and patience. Surfers have to earn their waves and don't get a free ride to the top, but once they're riding that wave, there's only so much they can do for 20 seconds, if they're even lucky to get that much time.
Which do you think is the most difficult among the three? Remember, all three of those disciplines, regardless of opinion, deserve respect unto their own.