Honestly, Blue Link. Red Link and Dark Link are fairly close to follow. I mean, red is my favorite color and all, but I've always liked how Link looks in a blue tunic. In various fan fictions I've seen, he is usually characterized as the smart one--which make sense... considering on the Triforce, blue represents Wisdom.
I've never actually gotten the chance to play Four Swords because I don't know anyone else who has the cartridge to multiplayer with, but I guess I'd have to find two other people to play with if I wanted to get the opportunity to play as Blue Link, who is player three.