Remember, we’re dealing with an alien species, so what we perceive as “charisma” might be totally wasted on them. Ever met a charismatic octopus?
I think it should be a scientist, and I would like it to be a woman. What about Lisa Randall?
I don’t think “charisma” is a good metric. Charisma is a measure of how much you get other humans to like you. But if you’ve spent your whole life charming humans you might have a lot of trouble understanding how to charm aliens, since your hu-mon tricks won’t work on them.
You’d be better off looking for somebody like Jane Goodall who’s spend their life studying non-human species. Or a semi-autistic person who doesn’t have an intuitive understanding of hu-mons and has to learn our social rules explicitly, rather than naturally.
So I just realized Temple Grandin is the obvious choice.
Myself.
I don’t think I’ll have any problems explaining Human Foibles and the bizarre thinking that permeates our species.
I feel like you’d do better with a science fiction author, or something.
Think of it like we’ve sent a few people to go meet with the Sentinelese people and explain to them the wonders of letting murders live their whole lives, having no idea who your neighbors are, keeping your personal possessions to yourself and locking them in your dwelling, and operating a political system where the leaders of the nation mostly can’t do anything nor order their subjects around. Woohoo!
The Sentinelese person chosen to talk to this group of people with “advanced” (or at least different) ways of living, morality, etc. doesn’t need to be someone who is a good and smooth talker. It needs to be someone who can listen and accept what he’s being told and envision the possibilities instead of rejecting it all off-the-bat.
Sure, some of that may be technological. But one person isn’t going to be able to understand all of the technology or learn it in such a short amount of time as to be practical. (Or, if he can, it would be thanks to alien technology that blasts the information directly into his head.) But that’s not relevant if every other thing that the aliens say completely disgusts and horrifies the sensibilities of our representative and he just immediately storms off in a huff, because the aliens are suggesting that our very way of life is completely wrong and that their, completely bizarre, obviously immoral, and crazy way of living is somehow better.
So we need someone who will listen and find this sort of stuff fascinating rather than horrifying, and accept that it may be an alternate possibility for the world. And then able to relate a vision of what it would mean, in a practical sense, for us. That is to say, a science fiction author.
Our goal shouldn’t be to wheel and deal with the aliens. It should be to figure out how what the differences in their and our societies are, and work to pull the applicable parts over into ours so that we can improve our world.
BINGO!
Or, the scientist of the hour, Dr. Nergis Mavalvala, who helped discover gravitational waves. I’d love to hear a dialogue between these two.
Much as I like scientists, no one scientist is going to come close to understanding their science and technology in reasonable time, because they are all too specialized. (Unless the Professor from Gilligan’s Island was still around.) A good diplomat could negotiate an agreement for the aliens to teach us.
I like the idea of Bill Clinton, except that he might be getting old enough to be losing it. The Pope is another one, especially if he returns from the meetings as an atheist.
Howard Stern*!!* 
Will the ambassador be leaving earth for the aliens’ home galaxy, never to return?
Donald Trump.
Who would be the modern day Isaac Asimov or Robert Heinlein? We’re going to want someone out there who’s not going to wind up with a massive inferiority complex curled up in a fetal position sucking their thumb in the corner.
And who knows enough to get the plans for a proper flying car.
Groucho Marx’s mustache.
It’s a real shame Terry Pratchett is no longer with us - he’s have been perfect. An inquiring mind, a decent layman’s grasp of science, deep understanding of human nature, and the ability to articulate it well.
I would go with Quentin Tarantino, figuring the aliens, being advanced, would be like, “Tarantino? The Pulp Fiction guy? Cool!”
Morgan Freeman.
James Burke.
The problem with sending David Attenborough is that it might set the Alien’s expectations too high. I would say set the bar low and build from there. Maybe Charlie Sheen?
I vote for Doug Stanhope.
As Emperor I anoint myself Ambassador.
[pedant]*and it’s spelled “thanks” [/pedant]
The problem is that we need someone with a deep understanding of Human nature. It matters not a whit if the person can understand the science. If they can’t explain Human irrationality and mindsets, we’re fucked.
Not in alien it isn’t.
I vote for George Noory. He’s listened to enough stories of aliens that probably nothing could shock him. He’s diplomatic–he has to handle lots of people on his show that have very odd views. He knows something about science–he has occasional guests who talk about scientific discoveries.