I mean, if we rank if by who gets a fight in each arc, then Sanji isn't a main fighter, as he got skipped in Skypiea (in a solo fight at least) while Chopper actually got a one on one fight with a priest. Although, I'm generally disgusted with the idea of main and support fighters, as all the SHs crew members are the main characters of the series. The support characters are those guys who visit and leave in each arc.
And I'm saddened by the fact that fighting seems to be the only criteria most One Piece readers use to judge whether or not a character is a 'main character' (i.e., an important character), as I'm sure Oda would be too if he were to read some of the comments here. Though I suppose that's to be expected of the fanbase seeing how the manga is, afterall, officially listed as a shonen manga. Knowing that, however, still doesn't stop me from lamenting the fact that too many are pigeonholing this manga as just a shonen manga, as if that's all this manga can ever be.