Who would it be?
OK I'll be honest; I was getting sick of all those circlejerk threads, so I decided to take the basic idea and add my own twist to it. Imagine this:
You and one other human being, who happens to be a TSR member, are immortal but no one else is alive. You're both infertile, so don't bother with the whole "we must repopulate the Earth" argument. Furthermore, you are always together; there's never more than ten metres between you, so they may very annoying.
Choose wisely.