Why, my namesake from SS1, of course. I just loved that behemoth of a sword he had in the first game. It was a pity that they gave him that stupid pillar in SS2.
Runner up is probably Tam Tam, followed by Haohmaru. I love Tam Tam's ending (try beating it with one credit) in SS1 and his stage is one of the best looking ones in the game. As for Haohmaru, he is blessed with the best voice acting in the entire series. He is also well-rounded and easy to play which makes him the perfect choice for novice and expert players alike.
Honorable mentions:
Charlotte
Genjuro
Ukyo
I know a lot of people disagree with me but I think SNK went downhill with this series after the first game. I played that game for almost a full year before moving on to something else. By the time I played SS2, it simply did not have the same impact as the first game. The third and fourth games also suffered from the same problem, hence the reason all my favorite are from the first game.