what is the best opener against the kings pawn?

Best for what?  Do you mean the safest opening response for black?  Or maybe the most exciting?  How about black's best attempt to unbalance the opening and create winning chances?  Without knowing what you want out of the opening, it's hard to recommend anything.  Or maybe you want an opening that you can safely get out of book quickly?  Let us know and I'm sure you'll get a few responses.

Oh, here's a few related threads.  Bound to be something of interest...

https://www.chess.com/forum/search?keyword=best+opening+against+the+king%27s+pawn%3F

I didn't mean to plagiarize, but the phrase "best attempt to unbalance the opening and create winning chances" is straight out of numerous opening books and articles.  They're talking about the Sicilian of course.  And just in case you've never looked at the Explorer...

I have always played the PIRC defence (d6 from above). In GM games the odds do not favour Black, as shown above. But, I found most beginners were unfamiliar with it and I studied it in a book by GM Yasser Seirawan (who no longer uses as often). I have had great success with it for those reasons.

The best openings and defences are the ones you are most familiar with and your opponent isn't. Good luck. 

Pirc - main line (Austrian, hardest to defend) = 1. e4 d6 2. d4 Nf6 3. Nc3 g6, 4. f4 Bg7

GM Yasser Seirawan on openings and the PIRC defence . . .

I usually respond with e5, sicilian defense. if Ng3 is played after that i continue with d6 and make a pawn chain. then Nc6.

there are lots of variations i play as response of different responses but basically it starts like that. i exchange pawns when d4 is played. if knight recaptures i recapture with knight and push responded queen back with e5. if bishop comes into middle with g3 knight i respond with h6 and stop an earl attack on f7.

You meant c5, not e5, of course.

im not good at remembering and reading coordinates, i was on phone and probably totally messed it up, ill write my variations now

1.e4, c5 2.Nf3, d6 3.d4, cxd4 4.Nxd4, e5

1.e4, c5 2.Nf3, d6 3.d4, cxd4 4.Qxd4, Nc6

1.e4, c5 2.Nf3, d6 3.Bc4, h6 (then knights on c6 and g6)

If white bishop pins your c6 Knight then Bd7 to remove the pin. If bishop captures then recapture with Queen.

If bishop gives check before you move your knight then directly respond with Bd7, and exchange if needed. If he backs his bishop up (Nc3) then push it back with a6.

I use that opening with a few more variations and it gives me a stable and balanced opening with a good pawn structure (a bit closed center if my opponent plays along). If pawn exchanges happen i keep my two central pawns while white has just e-pawn.

Then i try to castle kingside after i played Be7 and i'm ready to go in middle game.

Computer analysis usually tells me e5 pawn move is a bad one but i couldnt find a more appropriate, a better alternative push back move so far, so i use it

alehkhin'es defence