Black's best response to d4?

My Scandinavian Qd6 goes with a6 and possibly b5. But anyway what is Semi-Slav? 

I think I might reconsider my dropping on the Slav and taking up Grunfeld instead since everybody is recommending the Slav! Thanks everyone who did :)

as previously posted it depends what you are comfortable with however i think 1...e6 aiming for a dutch is good and flexible and also avoids staunton gambit and other anti-dutch lines. bottom line experiment and find what you like then stick with it so as to gain a deeper knowledge. 

What worries me is that someone of your calibur does not understand the positional nuanses of the 1.d4 positions and is trying different d4 openings to compensate.

The Slav, while not an easy opening per say, is the most fundamental of the d4 openings.  If you learn how to play that effectively, you can translate to a more appropriate opening to your style.  The Grunfeld on the other hand is very difficult to play.  The reasoning behind the moves is not clear, and the theory is in major flux constantly.  Heck even the top GMs today have a difficult time handling the opening.

The Slav is an important step in the development of a player, just as the Ruy Lopez is against 1.e4

1.d4 d5 should be your choice for now as Black.

Judging from your rating you should be concentrating on open games, although double QP openings usually lead to semi-open or closed games. As to what is the "best" defense to 1.d4 no one can answer that for you, you're going to have to find out for yourself. First you'll have to determine if you prefer to attack or defend and whether you prefer open, semi-open or closed games. The great hypermodern GM Richard Reti advised beginners to study open games first, and I think you should follow his advice.

In general the "best" openings are the ones that work for you most of the time IMHO and the "worst" openings are the ones you have the least success with. Since you play the Scandinavian Defense you might try the Albin Counter-Gambit 1.d4 d5 2.c4 e5. 

Long ago GM Larry Evans wrote a book The Chess Opening for You where he recommended the Scandinavian for Black against 1.e4 and the King's Indian Defense for Black against 1.d4.  (He also recommended the King's Indian Attack for White.)  The idea was that you can learn a general formation against 1.d4, 1.c4 or 1.Nf3 and you don't have to spend as much time studying hundreds of variations.  (Plus he also recommended the same formation for White.) 

For what it's worth, here is an example of the opening and how you might play it against several different White formations.  Maybe it's compatible with the Scandinavian and maybe not -- you can decide whether you like it or not. 

I played this opening for a long time when I was young and had good results at the time.

Hey! Thanks alot! That was helpful :)

Yes, you're right. I do not understand the positional nuanses of the 1.d4 positions and am trying different d4 openings to compensate. I guess I am wrong, and will start with Slav now :)

That's an interesting way to put it.  But what is the Scandinavian of the queen pawn openings?  The Dutch?  The Budapest? 

There is no such thing - "the best response to 1. d4". Forget it. You must follow your own style, that's all. If you like open positions (I assume you do cause Scandinavian usually leads to them), the Grunfeld is a good option, cause almost everybody goes to the classical variation 1. d4 Nf6 2. c4 g6 3. Nc3 d5 4. cd5 Nd5 5. e4 Nc3 6. bc3, with, usually, a lot of piece play. But there's a LOT - and I mean it - of theory to learn. Are you up to the task? Then go ahead! I strongly recommend you buy "The Grünfeld For The Attacking Player", by Bogdan Lalic.

However, if you don't have the time or taste for exhaustive theoretical studies, you must choose a defence which is a "scheme", such as the King's Indian Defence. You can use it against almost any White's "closed" setup (c4/Nf3/g3 etc.). Besides, as Black seeks for an attack against White's king in almost every variation, many White players don't like to play against it - after all, checkmate is the end of the game - no matter how many passed pawns and positional advantage White managed to get in the Queen's side meanwhile... I've won a lot of OTB games (and over the internet too) with KID. Despite its intrincacies, it's well worth to study this fascinating defence, specially the works of David Vigorito and Victor Bologan on it! But be aware that this is not an "open" defence, if you know what I mean - and of course it requires hours of study too... A good way to know with which of them - Grünfeld or KID - you will be more comfortable is to play them in blitz and bullet. By the way, I play both of them - maybe I'm an eclectic chessplayer? : )

I play the grunfeld a lot, but I seem to be getting a lot of drawn positions from it. Although my opponents will still often blunder away the draw, I'm not really liking it. From playing it I do agree that it is much easier for black to go wrong than white, so personally wouldn't recommend it to beginners. It also might not be the best idea in a blitz game unless you're really familiar with it.

Thank you my friend :) I dont wish to play the Grunfeld anymore. I think i'll look through the KID. Do you recommend any KID books? And just to tell you, i've never played a single blitz/bullet game in my life and I doubt i'll ever play in any blitz/bullet tournament in my life as I am perfectly sure i'll end up with 0 points :)

Given that you like Scandinavian:  Have you considered the Dutch Defense (1.d4 f5) or the Tarrasch Defense (1.d4 d5 2.c4 e6 3.Nc3 c5)? 

Ummm, maybe the Chigorin (1.d4 d5 2.c4 Nc6, or 2.Nf3 Nc6?). It has a few similarities to the Scandinavian, it's not too heavy strategically, and it's a sound opening.

GM's Morozevich and Miladinovic are empoying it pretty frequently, Carlsen beat Kramnik, Chucky beat Carlsen... not naive opening at all!

WHAT ABOUT ZIS?

Solid - Slav

In Between - Nimzo

Aggressive - KID

Slav and Nimzo are easy to learn for your reportoire while KID isn't too confusing after you understand the most popular lines. :)

 I just can't agree with that. Computers are strong but they don't always what is objectively best. Humans are still stronger than computers in many aspects of the game and a human should be able to churn out better opening analysis than a computer can. Even centaurs (computer aided humans), who are even stronger than the best engines, don't know for sure what the objectively best moves are.

Any idea where all the analysis is posted or is there a book on it?

Thanks :)

Personally I play the Chigorin (which is Nc6). You can see more about it here: http://www.chess.com/opening/eco/D02_Queen_Pawn_Game_Chigorin__Variation