Which one are you the most fond of? Are you a Street Fighter purist, or a Fatal Fury iconoclast? Do you rock the 3D games like Virtua Fighter or Soul Calibur, or are you more comfortable in the 2D paradigm?
Myself, I'm a huge Virtua Fighter and Soul Calibur guy. I play VF 4 or 5 an hour or so a night, and I used to participate in SoulCal III tournaments pretty regularly. My roots, though, are pure Capcom(Street Fighter II and III especially).
After a lot of thought, I'm going Soul Calibur. I love the guard impact mechanic, and the weapons add a lot of inter-character strategy. I've been playing a ton of 2D fighters lately, and while they have their charm, nothing is better for me than being able to side step a mouthy opponent and stab him in the kidney.