For me, it's a tough call between Yuffie and Vincent for my favourite playable character. I love Yuffie's youthful energy and childishness, and how she always held the rejuvenation of Wutai in the forefront of her mind no matter the situation. Then there's Vincent, who has such a cool persona and one of the most interesting backstories I've ever come across. Not to mention the fun / agitation of losing control of him during his limit breaks.
Hhm. At a push, Vincent, but I really do love them both.