I have a couple answers to this.

When Terry Bogard got announced for Smash, I knew who he was, but I didn't really know him. In all honesty, I actually thought he was an antagonist, for some reason. Once I played as him in Smash, though, I friggin loved him. This compelled me to look into his games, and now, I have a fondness for a LOT of FF/KoF characters. I guess I really appreciate him for that. Also, he's just so. Damn. Cool. Buster Wolf is one of the hypest attacks in the history of video games. I realize I probably sound like a basic *****, but c'mon, man. It's Terry.

My other answer is one no one else has given, based on what I've read/searched. That may be because he's not really from a fighting game, per se–at least, not according to most fans of the genre. I'm talking about Ridley from the Metroid series, specifically his Smash Ultimate version (not that he has any other playable versions, but I digress). He's a purple space dragon thing. He's fantastic. Granted, he's not particularly good, but he is fun, and that's all that really matters to me at then end of the day.