Most punishing opening against d4

The Gruendfeld is a good choice. One bad move by white, black takes over.

If there was a most punishing opening to 1.d4, it's pretty much dead certain that all GMs would be using it routinely and nothing but. Since this isn't the case, we can conclude that like with any opening move by white, black's choice of response is largely a matter of taste. At the master level theoretical soundness becomes more important, though below that it's almost entirely overrated in my view.

Bottom line is it depends what types of positions you're comfortable playing. Benoni, King's Indian, Grunfeld, Benko Gambit, Blumenfeld Countergambit, Albin Countergambit, Budapest Gambit, Dutch Defense are some of black's most aggressive tries. In all those lines white should still maintain a significant theoretical advantage with accurate play. But theory is very different from practice, especially for amateurs, and especially if you're talking games with fast time controls.
Queen 'Gambit' lines (of which there's no shortage of) including Slav- and Semi-Slav lines, Queen's Indian/Nizmo-Indian Defenses generally have reputations as being more solid and reliable, but also more passive/less dynamic. But again, theory is often completely different from practice'.

I've tried virtually all of the above openings and some other less talked about ones (Mexican Defense, Chigorin Defense), many only a few times against non-theoretically inclined opponents in blitz. I've been sold primarily on the Benoni and King's Indian for now, but that's me.

Until you learn to play a decent middle game though, opening choice doesn't even matter that much anyway.

WoW.

thats deep!

It depends on the player's individual psychology, experience, etc.  1...d5 1...Nf6 1...f5 are all good choices.  I like the Leningrad Dutch: you can move the queen early to e8, move a side pawn and then put a knight on the rim and then move the same knight again afterwards (admittedly, to primarily cover the e6 square weakness, but still.) Despite these seeming violation the mainline Leningrad Dutch lines are sound, and trying to "refute" it will put unnecessary pressure on the uninitiated.  Some lines even involve blocking the light bishop with a rook because of white's Ra3-Ng5 looking bad, but white's pieces are awkwardly placed and I even have some experience in this line (against a computer admittedly).

I didn't notice that!  I recommend nothing but at least three weeks of basic tactics for these kinds of mistakes.  Blitz isn't a lot of time to think so people just make moves based off their positional understanding and tactical vision for those situations that come up, and even then only the principles they know at an instinctual level.  It take about three weeks or so for a skill to become automatic too from what I've read.

vyik, that applies to most d4-c4 mainlines I think?

I like to punish my opponents for hanging pieces and leaving them tactically unguarded, but I doubt my choice of opening makes that any more likely unless I'm trading into an endgame right away. For the record I play the Semi-Slav against d4 and Caro-Kann against e4 and hate draws.

classical for me!

Pfren's advice, as usual, hits the bullseye.

But the question of what is the best line to play as Black in a must-win situation is interesting as a general question. My coach's response to such a question is to play what you know best. But I wonder what lines Pfren would play as Black against 1.d4 in a must-win situation.

I have to say, that whatever line you are most comfortable with/know the best will be most punishing. If you don't know the line, you're more likely to make mistakes with it (or be unprepared for any deviations your opponent might have planned), and ruin any advantage it could have gave you. 

(also, I find it amusing that your name is LethalLarry, and you've asked about most the punishing lines-very name approriate, no?)

"One thing that I love about my e4 lines, as both white and black, is that knowing them well can be a powerful tool for ending games quickly. One mistake for either side in the pp variation of the Najdorf, the Yugoslav dragon, the Evans gambit... They are unforgiving, but give you a chance to punish your opponent brutally. Also with the possible exception of a few gambits I play occasionally, these lines are super sound..."

So you are looking for the most tactical response to 1. d4 which is still sound, aren't you? I'm not an opening expert but I think openings like the Gruenfeld, The Benko gambit, or the Kings Indian might be the best option. 

I don't think the Benko gambit is very tactical

Didn't read previous posts, but I think "punishing" implies that one player made a mistake.

In that sense, there is no opening that "punishes" 1.d4, because 1.d4 isn't a mistake.

I can tell you, as a long time Queen's Gambit player, I disliked playing against the King's Indian the most. The Grunfeld is another interesting option. Of course you could always play a main live slav...

Again, Pfren is right. Below the master level, it doesn't really matter which (sound) opening you play. The tactics will determine the result.

At the very highest levels (+2650), the opening that gives Black the highest winning per cent (and also White) is the Budapest. But that is very rarely played at the highest level and usually only in blitz or rapid games. In other words, don't try the Budapest against a well-prepared opponent and expect to win. You will probably lose.

The King's Indian scores the highest winning per cent for Black among the main openings. Other openings that score particularly well for Black include the Benko and the Modern Benoni. These also give white a high winning per cent too.

This doesn't say anything about specific variations, which is what really matters. For instance, if White plays the Exchange variation of the KID, the game almost automatically ends in a draw at the highest levels. If you try that opening to get an easy draw as White and you're rated under 2300, expect to lose.

+Infinity.

Benkö or KID at club level

Firebrand, thanks for showing us that game!

In correspondence, the Benko has a pretty poor showing. I bet in ICCF correspondence it's even worse! (I dont' have a separate database with only ICCF games, so I can't verify that.)

Well said. I'm still trying to work on that last statement myself.

I'll add further that it's apparently bad practice to play when you're horribly tired...not a good way to win games, believe it or not.