Magus is a great character, he gets all level 2 spells and a lot of other great spells, and he flies! Robo is also cool in a way how he can fight well, heal and use some nice offensive techs too, and I like the story of how he's trying to find his emotions and the other robots consider him an outcast or a traitor. Maybe that's somewhat generic, but it was still well done, IMHO.
However, my favorite character was Lucca. You'll get her quite early in the game (unlike Magus), she has some funny moments in the game but also sad ones, and I think her fire techs are excellent. Flame Toss is great at the beginning of the game, especially when you combine it with Crono for the Flame Whirl double tech. Fire and Fire2 are also quite useful, Flare is a great spell and Protect is useful for physically weak characters during the final battle since the last boss can use Grand Stone (the ultimate physical attack, or something like that) to do 900+ damage to physically weak characters if you don't have Protect. And though she is pretty much a genius, I like it that she's still not really a mad no-lifer scientist character.