What are the absolute must-knows for a good hand as Black?  Sicilian and a good Closed Ruy Lopez are my cornerstones.  However, it hard to see which reply to 1. d4 is best.  Right now I use 1. ... nf6 and either play the Nimo-Indian, Gruenfeld, or KID, depending on their continuations.

How could the Sicilian and a Ruy Lopez be "absolute must knows" for black? Couldn't you just know one or the other?

 

I'm sure there is no "best" reply to 1. d4. Rather there are many good openings to choose from. None of them will get you a win, and none of them will make you lose. You get to do all that stuff on your own, that's why chess is so great. 

If you want to go off the beaten track and also minimize opening preparation, give some consideration to the Dutch against d4. Then it is you dictating the opening, not White, and you limit the number of lines you need to master by a lot.

The Ruy Lopez is something you should have mastered from both sides of the chess board.  If you know someone has a great game vs the Sicilian Defense as White, why wouldnt you play the closed Ruy Lopez?  Besides, getting comfortable with such an analyzed series of lines as the RL helps keep your play sharp.

 

 That's an interesting idea but the Dutch seems only fine and the fact that isn't highly played speaks to its dubious nature.  I think I just need more practice vs d4 but I just don't know where to focus my efforts.

Leningrad Dutch is very hypermodern. Kingside fianchetto, like the King's Indian. Korchnoi loved it.

Interesting.  I did a double take when I saw the opening line.  I'll check it out.