That's actually a tough one. Since you're not asking about main, which is decided by trying all the characters and deciding the one that has the best feel/clicks for me, but rather the one you try first.
And I really don't know.
I do know I will be staying away from Olimar with a 10-foot pole despite him being my Brawl main, actually it's because he is my Brawl main that I will stay away from him because I hate the changes.
Greninja, Wii Fit Trainer, Rosalina, and Mega Man all look fun to play. There's also Charizard now that he is solo. That's gonna be tough. I'm leaning towards WFT though.