I would say Luxord because his fight really, really blew, but Axel annoyed me as a character, and I was also horribly in love with Vexen for the longest time...
That said, a lot of people seem to have the wrong idea of Lexaeus. Just because his title is "the Silent Hero" doesn't mean he's not going to make a single peep and save little children from oncoming cars.
Nobody laugh, but I'm playing on Standard Mode cuz I'm a wussy-***** like that. That aside, does anybody know what exactly the requirements are to be able to fight him, in Standard Mode? Because I can't.
For the record, I've beat the game, have beat the Absent Silhouette fights, have beat neither the Org XIII mushrooms nor the Org XIII refights, & I'm at level 64 or so.