If White plays 1 d4, 1 e4, or 1 c4, I often play 1 c6, where White almost always ends up in a variation of the Slav Defense or Caro-Kann Defense, two of the Black Defenses I'm most comfortable where I know the ideas behind the openings and the kinds of middle games that tend to come from them.
You could also go be more straightforward and play 1 d4 d5 2 c4 c6 to play the Slav - this lets you get your c8-B to f5 or g4 instead of having it trapped behind your pawns in the Queen's Gambit Declined after 2...e6.
Which is the best opening for black when white played d4?