First of all, what kind of torturous question is this? What post-apocalyptic world would make you choose just ONE anime to watch for the rest of your life? Horrible really.
But since rules are rules, I would have to choose Naruto! So much energy, so much diversity, so many emotions, so many different characters to fall in love with and laugh, cry, and grow with. If there could only be one, I think Naruto could keep me entertained the best (: