The only characters I ever used regularly were Lucca, Frog, Crono, and Magus. Marle and Ayla and Robo were rarely in my chosen parties. I liked Lucca in part for her genious character, and in part for her sightscope, so she became a natural addition. Likewise I liked the knight Frog, cliche as he is, more than the cliche princess (what am I saying? They're all cliche... so I guess it comes down to chosing one's favourite cliches!) As time went on, these three were my most powerful. They got their triple tech together. At that point, reverting to the others would have meant a serious decrease in my party's power, so I kept with them.
Until Janus arrived. And yes, I call him Janus. I insist entirely that Magus is not his name, merely a title, which by the Mystics was used as a name proper for lack of anything else. This man casts magic better than us... he is THE Magician, so that's what we'll call him. The Magus. But properly speaking, Janus is not only his true name, but far cooler. It has that old Roman air, and cements him as the second most important character in the series, next to his sister: is is the beginning and the end, and the changes of the story revolve around the fortunes of his family. Not to mention, as Kato said, his is the 'heroic' character. He is, in fact, the most Classical of the heroes of the story. For him to die would be like killing Odysseus at the Cyclops.
And let me add a few things on him. Janus is not dark, per say. He is melancholy, contemplatitive, and sharp, but he is not a dark character, per say. He has not real trace of villainy about him, but he does what must be done. He is not someone worried about popularity, and never cares to have fun times. And here is one thing I find funny about people who think to identify with him... most of the time it's teenagers who identify with him purely from the perspective of being misunderstood or outcast. But these are not the most important aspects of his character. Indeed, I would say his high maturity, ability, and also his incredibly seriousness (in addition self-seriousness), make him who he is. Ironically, these are character traits most young boys playing the game would entirely lack. Very few these days consider being self-serious a virtue, and for all that they may be misunderstood, are typically goofy and outgoing, even if just to mask insecurity. Janus, on the other hand, has not a whit of insecurity, and if he laughs, it's bitter laughs, or derisive ones... but never quite comic. He is gloomy unlike any teenager, but rather has all the melancholy of a scholar (which, indeed, he is!) Just had to say those things about Janus. They ran through my head the other day. I think he's amongst the most mis-appropriated character in video games. So many people consider themselves 'like' him just because it's fashionable to be 'dark'... but bloody rot, unless someone is acting like a fourty year old with a cynical edge, they do not share much in common with him.
Anyway, he's not to die, either. He cannot die. He is, I would argue, the most important hero to the saga.
So for all that, I voted Ayla for the death.