What is the Most Aggressive (and Sound) Defense to 1. d4?

I'm looking for the best defense to d4 which gives Black solid counter-attacking chances.

 I think 1.d5. When your oppenent plays c4 you can play c6 or e6 and get into the semi-slav or slav.

I always preferred Slav and Semi-Slav. Found those to be the most aggressive responses to d4 from my experience...

My vote goes for the Nimzo-Indian.

To me, the defense must be sound with the possibility of imbalances in the position. I think the Nimzo gets that. The Slav, an opening I love, doesn't if white exchanges. It's the Achilles heel.

 The position is not balanced if white exchanges with the Slav or semi-slav? I never really found it so bad compensating for that... how do you mean, if I may ask?

I agree with Ozzie on both the Nimzo-Indian and his point regarding the Slav.  I drew Susan Polgar (ok so it was a simul) using the Slav but it just isn't as secure as the Nimzo

@PeaceMakerZero I mean that after 1.d4 d5 2.c4 c6 3.cxd5 cxd5 black really has to work to get some sort of an imbalance in the position. On the other hand, there are some more natural imbalances when black plays the Nimzo.

Apologies if I wasn't clear the first time around.

 white can also avoid the nimzo completely(to paraphrase someone who I cannot remember-playing the nimzo is not to everyones taste, not everyone wants an interesting game.  And this WAS NOT in a nimzo indian book).  

 Thank you, sir. I appreciate the clarification - sorry, it's been a long day :D

Benko Gambit.

1.d4 Nf6 2.c4 c5 3.d5 b5

I believe it is the KID.  It is very dynamic and is played by many of the world's top players.  Nimzo is also good, but can be avoided.

Personally, I prefer the Indian defenses, but the Slav and QGD are important to know too.

lol dutch defense try it you might like it

Is it possible to be aggressive and sound? :)

What do you play (that is both aggressive and sound) against 1.d4 Nf6 2.c4 c5 3.Nf3

Actually despite its possibly poor reputation, the budapest gambit gives black lots of play, and with good play black will get the pawn back.  If white insists on holding on to the pawn he will have some problems.

3...cxd4 4.Nxd4 e5 5.Nb5 d5!?

GMs still play it, so I would guess it is still sound.

I personally like queen's gambit accepted. It opens up the center allowing black's bishops to be more active.

How about "1.d4 Nf6 2.c4 c5 3.Nf3 ----> 3...d5 = "

Has anyone tried the Kurjatko Gambit 1.d4 Nf6 2.c4 b5 !?

I tried a couple and found it highly entertaining and thought provoking, tho I don't play it especially well myself. I do know someone who played only that opening for several years of his online play, and got up near Expert with it. Even tho he would play either side of it, and so might find himself competing against his own prior ideas at times, since all his games with it were accessible online to his opponents.  So personally, I think it tested well for him.

I know a lot of people that play that line because it is sort of tricky and matches up in style well with the impetus behind playing 2... c5, but I wouldn't call it aggressive and sound in the same vein as the Benko.