I play c6 against 1. d4 in hopes of reaching slav or c-k
Black's best response to d4?

I play 1..d6, which almost always ends up in a Pirc. I also play 1..d6 against e4. In both cases, white almost always pushes both the d and e pawn out two squares:
1. d4 d6 2. e4 Nf6 3. Nc3
or
1. e4 d6 2. d4 Nf6 3. Nc3
This gives you the advantage of playing the same position against either e4 or d4, assuming you like modern setups.

Personally, I always respond by either e6, c6or g6.
If you're looking to disrupt preparation, g6 is easy. It's also good because then you only need to learn one opening, because g3 with white has similar pawn structures and very similar strategies.
If you have no preparation, e6 is the easiest to play
Equally possible is the Dutch:
d4 f5!
Almost nobody prepares vs the Dutch, but as Yusupov and Nigel Short proved, it isn't unsound
If you want to prepare the Dutch, go for the Modern Stonewall
Have fun

Try 1...e6 which can to transpose in french defence or after 2.c4 Nf6 to catalan nimzo indian or even bogo indian.Also with 1. ..e6 you may also have a QGD game...or a semi slav

leningrad dutch with black response to d4- f5 then g6 and placeing the black bishop at g7 - aggresive and unexpected

"blacks best response to d4?"
may as well ask:
"what is your favourite response to d4?"
Almost no one gives objective recommendations.
Im not 100% up to date on cutting edge theory, but I believe blacks best tries to equalise in the mainline atm are the Nimzo, semi slav and Grunfeld (current status of exchange lines??). The QGD can also be thrown in as solid, but I think white can turn out an edge in the exchange lines, and maybe the mainline (?)
The answer is very much a personal preference. I enjoy playing the Dutch, but Kings Indian and the Gruenfeld are fun too. It really is more about playing your best and not winning unless your a professional trying to make a living.

The answer is very much a personal preference. I enjoy playing the Dutch, but Kings Indian and the Gruenfeld are fun too. It really is more about playing your best and not winning unless your a professional trying to make a living.
Not really. ''Black's best response'' can be quantified. It only becomes personal preference between a few openings which qualify as black's best tries.
Below professional level it's true almost anything can be played, and that is personal preference. But just because someone prefers a dutch over a nimzo indian does not make the dutch an equally good opening against d4.

Below professional level it's true almost anything can be played, and that is personal preference. But just because someone prefers a dutch over a nimzo indian does not make the dutch an equally good opening against d4.
That depends what you mean by "best". If you mean best in a theoretical sense, having studied all top-level computer analysis in detail, then what you say is probably correct.
But to any sub-2700 player this definition of "best" isn't particularly useful or helpful. I regard the "best" move as the one most likely to win the game.

OH what a horrible thought that there is ONE best answer. That's far from the pleasure we get from muddling through a game surely?!
'Best' in terms of what? Scoring a point is only one of the reasons we play chess...;)
Even that very scienfic statistical definition will give you an answer that is temporal. Constantly changing as new ideas arrive, top players adopt different openings etc.

It's a classic garbage in, garbage out question, but if black is happy playing the French 1...e6 gives quite a lot of extra options for black. Some transpositional, and other completely independent lines.
Not sure would view ICCF games as the source of best opening play. The hardware, and software being used will have a much bigger affect on result of games, than the choice on move one. In a sharp, concrete lines, like Nadjorf, software can refute certain lines. On other hand don't think software will tell you very much about Berlin endgames, and what Carlsen is playing there, is likely to be more important.
First, if you find White not following up 1.d4 with 2.c4 in most cases, then that person doesn't know the idea behind 1.d4. Yep.
A43: Old Benoni Defense - 1.d4 c5.
1. It can take White right out of his comfort zone, immediately.
2. The d4 systems are disrupted,i.e., London, Colle, and the Torre Attack.
3. It can transpose into the Modern Benoni after 1.d4 c5 2.c4 Nf6.
4. White's most popular response after 1.d4 c5 is 2.d5, but then he needs to be comfortable with 2...e5. After 3.e4 d6, the center is locked and both sides will be looking for play on the flanks.
1.d4
Black could try 1...d6, with the idea of playing the Pirc Defense after 2.e4 Nf6.
I usually play 1...Nf6, preventing 2.e4, which would give White the ideal pawn center with d4 and e4. After 1...Nf6, depending on what White does, I can play the Nimzo-Indian Defense or the Modern Benoni Defense.
E20: Nimzo-Indian Defense - 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4
A60: Benoni Defense: Modern Variation - 1.d4 Nf6 2.c4 c5 3.d5 a6
A57: Benko Gambit - 1.d4 Nf6 2.c4 c5 3.d5 b5
One plan with the Benko Gambit, after picking up a tempo for the sacrificed b-pawn, is to quickly transfer the h-rook over to the open b-file and attack queenside. It's fun.