Personally, I'm voting the Massif Bros. I mean come on, their beef related jokes and overly stupid comments are hilarious, and one of the best parts of the whole game. You can pretty much define them in one picture:
I must say Big Massif and Lil' Massif. Even though I don't really like the kind of humor found in the Mario & Luigi series (well, I only played Dream Team, but I'm pretty sure the rest of them have a similar humor), I must admit these two guys were kind of funny.