But in seriousness. I would become immortal but with some restrictions so I don't go mad:
1. I will specify infinite youth, not making the same mistake as Dioysius
2. I will die once all sentient/sapient life in the universe is gone, if humanity is wiped out and I am still alive this could prove the existence of extraterrestrials which I will eventually find, possibly even other immortals, we could keep eachother company for eternity.
3. Be able turn off my immortality and die whenever I want as a last resort if the immortality DOES drive me crazy.
It would also help if I got a side-ability of being able to create any matter I want, I could create a new world to stay sane in case the dark gets too lonely