I'd pick a large mosquito, immortal no matter what you try to do to it and there forever, torturing people for all eternity
If that's a no, I'd honestly pick me, losing people means nothing to me since I no longer am able to feel most emotions at all due to medical reasons and I can always find something to keep myself from getting bored with, plus money wouldn't be an issue since turning into a soldier of fortune while immortal is just easy cash! As for when the universe dies and everything, if cryogenesis can pause consciousness, then I'll drift through space in an ice block like Kaars from JoJo until I end up somewhere and thaw out, then find something fun to do there, nobody knows, but I'd rather take the chance with immortality than die, ESPECIALLY if heaven and hell are real and you go there post mortem, heaven sounds boring AF to be for all eternity, hell is just a pain in the ass, pun intended...