I tried SSB once and couldn't make heads or tails of it. So many good choices in the poll, but it says favorite, and so I've got to go with Soul Caliber. Yeah, the last two were awful if not gorgeous, but I simply adore the variety of exotic themes and styles each character brings, from the caveman, to the mummy, heck, it's got pirates and ninjas too!
If the question would have been greatest, my answer would have been a very different one, since I'm pretty sure there's no doubting the series that really popularized the genre to ridiculous success. If not for SF2, the landscape of fighters would not be what it is now.