You are given the task of bestowing immortality on exactly one person. Whom do you choose, and why?

Someone loathesome who deserves the misery of living forever, such as the Supreme Leader of North Korea, Kim Jong-un. I assume that he will be aging normally.

You – so you can witness the demise of the USA,

I would pick the worst jerkwad I have ever met. Immortality would be a curse, not a blessing.

I would choose a sweet and intelligent child, hoping they have the right character to use the time wisely to learn, grow, and do well for the world.
The restrictions are fine by me, as I would not choose anyone who has put me in their will.
It looks as if it is unanimous.

You already know there can be only one.

Myself. And if I can’t bestow it upon myself then fuck it, nobody’s getting it.

Lhamo Dondrub (Jetsun Jamphel Ngawang Lobsang Yeshe Tenzin Gyatso) Just to piss off the Chinese Government.

And probably most of Christianity but that would be an unintended side-effect

I’m with @Darth_Algar, if I can’t live forever, no one can.