Best Response as Black to 1.d4?

Lately I have been playing the QGD.

polish defense

If you play the French, then why not 1. d4 e6? If 2.c4 you can decide on Indian 2...Nf6, Dutch 2...f5 or QGD 2...d5.

If you play the french then I'd recommend the english defense. You can play the horwitz defense 1... e6, and if 2. c4 b6. If 3. Nc3 Bb7 english defense, otherwise if 3. Nf3 Bb7 4. Nc3 either Bb4 moving toward the nimzo, f5 the dutch, Nf3 the QID... The beauty of the english defense is it's rare, can be very aggressive, and it's alot less theory than the nimzo. And then of course if 2. e4 d5 you're back in the french.

Otherwise if you want something a little more standard go with the nimzo indian, but I'd still play it via the horwitz move order 1. d4 e6 2. c4 Nf6 3. Nc3 Bb4. The horwitz opens up some interesting and uncommon options against a number of whites alternative moves, like the zuckertort or london.

i.e. 1. d4 e6 2. Bf4 b6 or c5 
1. d4 e6 2. Nf3 c5 - can lead to some french sicilian transpositions and a few other lines (not a requirement but if you're up for it it's a cool system)

Benino defense

I would recommend playing 1...Nf6. There are many things you could play. For example if they play 2.c4 just play 2...e6. Or if they play 2.Nf3 just play 2...g6. Or if you don't want to play 1...Nf6 then play the Slav or Queens Gambit declined. But there is no one best opening against 1.d4. There are many options.

There is no "best" defense else every top GM would play it. There are a lot of strong defenses Such as the Grunfeld ( the one I like), the nimzo and kingsindian and several queensgambits and slavs. Even the Dutch defense is pretty much okay till you become a top GM

For French players the Kings Indian defense is a decently logical companion as they in a way both have the same strategic basis in pawn chains. However at ~500 rating you probably have a better result with the Queens Gambit Declined and just develop your pieces as you wont get the KID on the board because white lacks knowledge to play it.

@spectros1

As a French player, you can also start with 1.e6 against 1.d4, because if they play 2.e4, then you're back into a French. If white plays 2.c4 (or 2.Nf3, for example), then I recommend you play 2.d5 and learn how to handle QGD structures with the black pieces.