Well, I voted Robo. He does have some pretty hilarious moments, and he is the one that started that whole Entity discussion, but he just isn't as appealing as the rest of the characters.
Honestly, I'm confused as to why Ayla, Marle and Magus have so many votes.
I was five when this game came out. Ayla seemed like the best mother figure in the group, especially after her talk with Marle over the rainbow shell.
As for Marle, RPGs just seem wrong to me without a good supporting caster. Storywise she's loud, forceful, not afraid to help anyone and everyone, and she grows a lot over the course of the game. Add to that the fact that she seems to instigate a lot of the pivotal moments in the game, and you get a character who's too important to whack off.
And as for Magus, well, he seems pretty obvious to me. He is The Magus. He duped the Mystics since childhood and managed to get a psuedo-religion built around him. He made Glenn into the hero the world needed (face it, Glenn would still be a marshmallow if it weren't for the loss of Cyrus). And after the party spent so long trying to kill Magus, he still offers up information to revive Crono. Besides, if you don't take the time to grind to level ** and farm tabs, Magus dishes out more damage than anyone at 2000+ per monster per attack. No faster way to beat the game than popping on a Gold Stud and Dark Mattering your way through the Black Omen.