From first row, my vote goes toward Banjo and Kazooie. I'm personally more familiar with them, so there, unfortunately, is a bit of bias-y here, but if we put that aside, I believe much of us could agree that these two are very vibrant and jolly characters, and would probably be more suitable as players of SSB (since, after all, this is how a majority of characters are portrayed--bright and colorful).
Second row, it's definitely Protoman. I've been a fan of his for an extremely long time, and still am. What, I believe, would make him a fun player is that he's seemingly a Samus-Link hybrid, and those who are fond of these two brawlers would likely enjoy playing as Protoman as well. Basically, he's the best of both worlds.
As for third and fourth, I don't know much things about those guys, unfortunately, but if I were to choose randomly, I think I'd select Isaac and Skull Kid.