First I read this as "which touhou would you rather be". If that was it, my answer would be rather different. But if it's only ability, without any of the personality or physical attributes, the answer is easy.
Imagine never having to live with any regrets, why you did this, or why you didn't do that. Never having to wonder what could've been or what could've happened differently. And maybe best of all, not in any way limited to yourself.
Although - and I'm surprised it hasn't been mentioned more yet - an awful lot of responsibility would come with an ability like this. But if you could just stand that, life would be great.