See, I come across this question ever so often. And I think it is kind of an abused one for the following reasons.
1 - A lot of users will point towards powerful characters that they like. If someone really, REALLY likes Kenshin, there is a high probability that he would be picked. Despite there being a laundry list of characters that would simply pick him apart without considering it to be a warm up. Again, the question is fundamentally not about likability, but likability often clashes with our judgment on the question. The medium anime, has created so many vastly overpowered 'god' characters that it is not even funny. Picking from them would be choosing between apples and oranges really. I mean, Haruhi has reality warping powers, which probably means she would beat a lot of really cool characters, that have power levels that are more justified compared with their characters.
In these examples, I always like to take Fist of the north star as an example. They feature a lot of very powerful, but very mortal characters. Now, strip down the story to the bone and there is not a whole lot, the characters however, are amazing, and carries what would essentially be kung-fu violence without that aspect to the series and ascends it into the stuff of legend. Watching some of these personalities clash against each other, in what is nearly certain to be a combat to the death. No quarter given, no mercy shown, is something that has inspired an entire genre, yet which no one has managed to replicate entirely in my opinion. And it is also a lot more interesting than pitting god X against god eater Z. But it is unfair to pair this up with said immortal characters that could, with a flick of their wrist, demolish said characters. It would not be remotely exciting, and feel very cheap.
So the best way to rephrase the question would be "What's the most powerful character that is justified in his own strength?" That would produce much more interesting results, and much more fun forum rages. So that's my two cents.
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