But he is still called Luigi, (the game actually calls him Luigi)Anyway, the Mario games have always been my favorite. I guess for argument's sake I'd have to pick Mario though since Luigi really isn't playable in anything but Mario 2. (I don't count the 2nd player in Mario 1 as Luigi).
I really don't think that Mario could have been successful without Luigi, back when he was in the DK arcade he really wasn't much, just a plumber running up some sort of metal structure chasing a monkey and a princess. Mario would be worthless without Luigi and I think that Super Mario Bros really opened up this concept officially, (though the Mario Bros Arcade game did come out first and Luigi was in that.) I just can't stand the fact of Mario and no Luigi.