Fire Luigi 4 life!
(white and green, in case someone is too silly to know)
I love all of Luigi's costumes, but Fire Luigi is a true classic of an alternate costume in this person's point of view. It isn't just brand new colors invented for the game, or someone else's colors slapped on like the (very awesome) Waluigi costume is. It's a genuine style that Luigi wears in his games, so it doesn't feel as cheap to wear it and if you're a Mario fan, you no doubt will be attached to that beauty. On that subject, it's a good moral booster to know you're the wicked-awesome Fire Luigi as well, which speaks a lot more words than his other choices can.
The only way you could be more classy is with a model swap, but since we can't have that, let's just stick with the canon color swap. On that note, Flying Luigi from Galaxy would have been great too, but maybe if there's a Smash 4...?
Funny story as well, I wanted to see an Orange Luigi and a Waluigi Luigi in Brawl, back when he wasn't even revealed as a character... it was awesome to see those colors when they were eventually leaked, let me tell you. Feels good, man.