I’d be happy with eternal youth. That is to say, physically staying the way I am now, my body not deteriorating and eventually collapsing.
I have so many things I want to do, so many books to read, languages to learn, martial arts to master, sciences to study, people on the internet to enter pointless disputes with.
I wouldn’t want to be immortal, though – if I ever get tired of life, I’d prefer to be able to stop.