Here is a basic list of established religions. While I am sure there are more comprehensive lists out there, This will give you an idea of what is meant by established religions.
There’s a pit thread about simple questions with straightforward answers thjat links to this one, which is how I read this thread. I started answeering this thread in the other one, then pasted my response here instead, but left in some confusing language.
This is exactly the opposite of the way I usually consider religions. My usual way is to look at them and consider how true or useful they are. The idea of choosing one because that would make it true is a mind-bender. If I were actually being given the choice, I’d ask how long I had to research the answer, because I’ve never gone looking for the-best-relition-if-only-it-were-true. It might take awhile to find.
Fantasy religions have been disallowed, so I can’t choose the pantheon of Discworld. I have no interest in living on a disc, but I like the ambient magic and love the fact that gods grow or shrink according to the belief in them. It gives a kind of leverage to us poor mortals.
The closest traditional, established religion that I know of is Shinto. It has kami, which have their own life cycle and whose effects can be mediated. And it affects the kami with, among other things, ceremonies that include music, song, and dance.
There could be something I’d like better, but if I had to choose quickly, I’d go with Shinto.
Found it and posted, thanks, Boyo Jim.
And now that that is done and it turns out I am somewhat observant after all, am I the only one who finds the list Czar posted somewhat ironic?
I’m curious as to why many of those who chose Buddhism specified Zen. I mean, was there a particular reason for singling it out or is it just the best-known variant in the US?
Well, I wouldn’t choose Buddhism. Who would want a reality in which life was suffering? (it may be true, but one would really prefer it was not).
Taoism would be okay I guess (again I suspect it is probably at least somewhat true, but that isn’t the point).
The Greek or Roman gods are right out - really nasty (not to say unlikely, even for gods). Ditto most other pantheons.
Don’t know enough about Shinto to comment, but I suspect you would have to be Japanese.
I would not choose Judaism, because that version of God strikes me as unpleasant (though again, if there was a god, I imagine he would be).
Islam has a nice paradise, which is a plus; but otherwise suffers from the same problem as Judaism.
I would probably go with some form of mainstream protestant Christianity. God is relatively inoffensive and nice and Jesus is a personal saviour- that would not be so bad.
If the OP had read “What kind of God would you create if you could?”, you certainly would have had to interpret it to mean that.
But it didn’t, so you don’t.
Zoroastrianism. They could use the boost.
Taoism
I am offended by the idea of including atheism among established religions.
But ok, that’s what I would choose.
Which means, should I choose atheism, the voice giving me instructions would have to eliminate itself as a factor in the universe, and absolutely nothing else will change. The incident would be entirely consistent with me having some kind of brief, non-recurring schizophrenic break. Making such wholesale changes that would allow the whole word to suddenly understand there is no God would require the kind of divine powers that my world wouldn’t include. .
If you believe in a religion, and decide to impose the dogma of your religious faith as the reality of the entire world, then you are pretty much certainly to be changing the nature of God, or adding or deleting an existing god, as far as much of the rest of the world is concerned.
Which leads to another question asked earlier. Would the rest of the world understand that the change had been made, and fall in line with the tenets of the new faith? Or if not fall in line, than understand the the punishments for disbelief or sinning, whatever they are, will actually be applied against them. IOW, will this process remove humanity’s doubts about religious faith?
Check your messages.
OK. I will go for Christianity, more precisely Anglican Christianity, even more precisely my own brand of Broad Church Anglicanism with a side order of the doctrine of Universal Salvation. (So there is a heaven and a hell, but nobody needs to go to the latter.) God is Love, love God and love your neighbour … can’t see any problems with that, really.
Unitarian Universalism, and BTW, Voice From Above, how do I know you’re not a hallucination?
If it’s a hallucination, there’s no harm in answering. 
I’m biased toward the method I practice. Can’t speak for anyone else.
Shinto has its appeals but:
(1) It doesn’t have one God – there are supposed to be 8,000,000 kami, where “kami” means spirit just as much as god – so it probably does not satisfy to OP.
(2) It is very much limited to Japan.
(3) If it were established, the Japanese emperor would be definitely the descendant in the male line from the sun goddess Amaterasu, so would be semi-divine himself. This might speed up Japanese plans for world domination.
But if Shinto could be made more universal, then I, as an atheist, might support it. But if not, then Buddhism.
Atheists say all the time that atheism isn’t a religion. And now you pick it when asked to pick a religion? Way to go. 
Heh, I’m somewhat surprised by the popularity of Buddhism - the Four Noble Truths reflect a somewhat depressing reality, if one really gets to choose. 
Could you provide links to the two so we can see what the difference is? I’m curious now.
On the plus side, if Shinto becomes the default ultimate truth doesn’t that mean the rest of the world gets to contendedly be atheist?