Ken is the best. And yes, the OP is a loaded question that is meaningless. No one wants to know who would win 1v1 at their peak abilities since it is pretty clear what would happen. Heck, the 30th best player may beat Ken if they played today (as Kishprime notes). In 4 years (if M2K stops playing) the 30th best player would beat M2K.
No one wants to know the answer to that question. Everyone wants to see who was comparatively better for their times and who had a better career. Ken wins hands down.
A question isn't loaded unless it intentionally tries to push for a particular conclusion that isn't a direct conclusion of the answers itself. For instance, if I said that M2K is the better smasher, or more successful smasher, because of the responses to my question, then my question would have be loaded. But I didn't make such a leap in logic. You say "no one wants to know" as if the question is redundant and obvious. I agree. I feel like the OP question is dumb as hell.
But I made this topic because people literally debate about it, and I made it knowing that the question is so dumb that everyone would answer the same way (at least, those who answered my question and not their own questions). This way, I have the voice of the community against the naysayers who actually believe Melee is the kind of game where the meta changes very little over 3-4 years.
These naysayers are the ones who
do generate the loaded responses. They take Ken's career success, and make the illogical extrapolation that it implies no one will ever be more skilled with Marth than he was. This is how they think, and this topic and its responses are my clear-cut refutation of that.