Luigi for sure. Not only can he do all Mario does and more, but he can also jump higher and farther. That's right, he's a better jumper. Jumping. The thing Mario is "known" for. Luigi's better at it. Yeah.
Also, I've always preferred the color green over red. I like earth tones better, I suppose.
Finally, Luigi's starring role in the web-series "There Will Be Brawl" has made me a Luigi fan all the more so. Even if Mario is portrayed as a badass beserker. :-P