While I am willing to do so for all 69 of the Generation VI Pokémon, I cannot do so tonight, as it is past midnight here and I am getting tired. I will be going to sleep soon. I will try to do as much as I can tomorrow though.
But if anyone else wants to do so, I can show you how to do it tonight! Then, while I'm sleeping, if there is a Pokémon that you are just dying to know the stats for, you can calculate it.
First off, the formula for stats is:
The source code of the Pokémon Universe wrote:((((a+(2*b)+((c)/(4))+d)*e)/(100))+f)*g
a = IV
b = Base Stat
c = EV
d = 100 if the stat is HP, 0 otherwise
e = Level
f = 10 if the stat is HP, 5 otherwise
g = 1.1 if the stat has a beneficial nature, 0.9 if it has a detrimental nature, 1 otherwise
We know that the formula is not changing, because if it were, then it would not be possible to transfer Pokémon from Generation 5 to Generation 6.
Now, first of all, you need a leaked screenshot of the Pokémon that you want. It must be on the "Summary" screen so that you can see its stats.
First, write down that formula above six times, once for each stat.
As far as Nature is concerned, any stat listed in white is neutral, blue is a detrimental nature, and red is a beneficial nature. So that should tell you the value of "g". Plug it into each of the six equations.
You can presumably see its level, so you can plug that in for "e" as well in each of the six equations.
"d" and "f" are given to you by the formula, so you can just plug those in right away.
EVs and IVs are tricky. With EVs, you NEED to know how many the Pokémon has, so check its Experience Bar. If it has ANY experience, then tough luck. It likely has EVs, and you presumably don't know how many or of which type. Worse, with Super Training being a thing, it means that even with no experience, it's possible that the Pokémon will have EVs. If this is the case, then it may screw up your calculations. But it will always screw them up by making the base stats TOO HIGH. So you'd at least end up with a maximum, knowing that it is "this value or lower". Plus, I don't think it's unreasonable to assume that people take screenshots before doing any Super Training.
So assuming the value is 0, set "c" equal to "0" in the formula.
And now we get to IVs. This is impossible to deal with because IVs are not given to you in-game, nor is there any way to calculate them without knowing base stats. So what you have to do is set "a" equal to "0" for your maximum stat, and set it for "31" for your minimum stat. You will end up with a range of values, with the base stat somewhere in there.
With this in mind, you have now plugged in a number for every variable except "b", so you end up with an expression with only one variable. Set the expression equal to the stat shown on the "Summary" screen and then you can solve for "b". When you do this, you get the base stat. Do this twice for each stat, once for the "0" IV and once for the "31" IV, and you get a range of possibilities for each of the six base stats.
So that's how I calculated Zygarde's stats. And I intend to calculate others's stats in the same way if I can. But if you'd like to do so, here's how it's done.
Oh, and if you don't feel like doing complex math, just substitute the numbers and go to Wolfram Alpha, and it will solve the equation for you.