Best reply to 1.d4?

Ppl  always talk about Kings Indian, semi slav etc but I'd like to know what is the best reply to 1.d4?

Whatever you want it to be...

I usually play:

D.  e6

A., B., or C. Is move two for me.

I think that it is fairly common for a player to be advised to first learn the Queen's Gambit Declined. If nothing else, it is probably going to be easier to find recent introductory opening books like First Steps: The Queen's Gambit

https://www.newinchess.com/media/wysiwyg/product_pdf/7652.pdf

and The Queen's Gambit Declined: Move by Move.

https://www.newinchess.com/media/wysiwyg/product_pdf/7743.pdf

It might be a good idea to consider Discovering Chess Openings by GM John Emms (2006).

https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf

More advanced:

Playing 1 d4 d5

https://www.chess.com/blog/pfren/playing-1-d5-d5-a-classical-repertoire

https://www.qualitychess.co.uk/ebooks/Playing1d4d5-excerpt.pdf

Play 1.d4 d5 2.c4 e6!

http://www.chess-stars.com/resources/contents_Play%201.d4%20d5%202.c4%20e6!.pdf

These are some of my reply to d4.

1. d4

2. Nf6

3. g6

4. e6

Best opening for black vs d4 is Nimzo-Indian Defence (1.d4 Nf6 2.c4 e6 3.Nc3 Bb4). That's why some players play 3.Nf3 to avoid Nimzo. Also there is video on youtube Best answer on d4. Reason - this opening breaks popular plan for white : d4, c4, Nc3, e4. 

There is no best defence.

If it was the case, u would see it at top level every time.

We see Berlin defence against 1.e4, also Petroff, for some reason, but it is on 1.e4, and it is also fashion so it doesn't mean these defences are the best.

On 1.d4, we didnt find yet...

Anyway, the question doesnt make any sense : u have to make your own choice, to play the variation(s) u prefer.

Thank you all for the extensive response! I feel much better already.

One question to Cerberus283: what means GGD?