Definitely Link. He was the very first character I tried in Super Smash Bros for Nintendo 64, though I eventually ended up maining Ness.
Though Smash 64 was the first Smash game I played, the first game in the series I actually bought and owned was Super Smash Bros Melee. I had gotten into Melee simply because it was a sequel to the original game. The first character I tried in Melee was Ness, but over time I ended up maining Marth.
When Super Smash Bros Brawl was first announced, I was thinking about skipping the game, as at the time I was a little tired of Smash Bros. But, after learning Pit and Meta Knight, and later Sonic, were going to be in it, I was immediately won over. In Brawl, I had planned to main either Pit, Meta Knight, Sonic, or Marth, but it was ultimately Zelda whom I stuck with.
[EDIT] Oh, and as for Smash 4, I was considering skipping that one as well, but after learning tripping was removed, I decided to give it a chance. There is no particular character that is motivating me to buy the game. Even if none of mine most wanted newcomers make it into the game, I probably will buy the game anyway. Though, I would be very pleased if Isaac made it in.