Most dangerous system by white (Catalan)

I have played many correspondence chess games at many sites and have come to the conclusion that the most dangerous system to play against is the Catalan system. I think that all opening moves by white other than e4 and d4 are objectively inferior. However after 1. e4 e5 white's best shot is still the Ruy-Lopez which can be defended against by the Berlin defense in an absolutely logical and foolproof way (if you know what you are doing). But I have found no such foolproof defense against the 1. d4 opening in which case black can do very little to prevent white from going into to the Catalan build up ( pawns on c4 and d4 + fianchetto bishop on g2 and no knight on c3 ). White will either have space advantage due to the pawns on c4 and d4 or if the pawn on c4 is taken black will have problems in the center. The ice is very thin and even lines offered in good opening books can lead to disaster. Currently I think that the best response to 1. d4 is not 1. ... Nf6 but 1. ... d5 and after 2. c4 and some form of Queen's Gambit declined ( I prefer playing e6 on the second move rather than c6 ).

Not the most dangerous,  though quite dangerous.

I'm not a coach or even titled, but I want you to go on a project: Study 10 games (at least IM level) where white lost with the Catalan. 

I'm not saying that you can't lose with the Catalan. You can lose with every system if you don't understand its underlying ideas. Kasparov lost against Judit Polgar with black playing the Berlin defense. This does not say anything to me about the Berlin defense which I still consider the most stable (Kasparov in that game made mistakes that are even obvious to me, this only proves that he did not fully understand the dos and don'ts of this defense which he played very rarely rather than that the defense was compromised).

I only say that if played by a strong player having full understanding of the system I'm most insecure playing against the Catalan.

If the catalan gives you fits maybe give the Slav a try.

As an practical opening, it's pretty dangerous if you use this system consistently, and you're well-versed in playing w/an fianchettoed bishop.

I disagree, though, that 1. ... Nf6 is weaker than 1. ... d5. It's an flexible move, and quite often transposes into QGD, QGD: Tarrasch, QGA lines (after 2. c4/Nf3 d5) as playing the KN to f6 is essential to many QP defenses, mainline or not.

The main idea of the Slav is that at some point you take on c4 then play b5 supported by the c6 pawn. I have tried some of these lines but they seem very risky to me.

It seems to me that 1. ... Nf6 2. c4 e6 has added value over 1. ... d5 2. c4 e6 only if white plays a knight on c3 that can be pinned. Since this can't be gauranateed I go for 1. ... d5 right away.

"Currently I think that the best response to 1. d4 is not 1. ... Nf6 but 1. ... d5 and after 2. c4 and some form of Queen's Gambit declined ( I prefer playing e6 on the second move rather than c6 )."

But 1...d5 does not stop white playing the catalan. And you also have to have clear understanding about the middlegame positions that may arrive. But the big practical advantage is that this setup can be played against almost anything.
 

The Catalan can't be stopped at all. There is nothing to prevent white from playing c4 and then fianchetto the bishop on g2. This is my problem. Currently the best I can come up with against it is d5. Not to stop it but to contain it.

Whenever i play against the Catalan, i opt for the Open variation (1. d4 Nf6 2. c4 e6 3. g3 d5 4. Bg2 dxc4 move order). For me, the main idea is to deprive or swap the Bg2 of it's strength on the long diagonal. Most of the ideas in using this "dangerous" system is Queenside play and skirmishes in the center. Knowing this is helpful, because if you're solid in the center, you'll be able to mobilize your forces to the Queenside for some in-square combat by forcing white to spend some tempi to regain the c4 pawn.

The essential idea in any QP defense isn't to just pin the QN but rather to influence control and occupation of the e4 square by white. Therefore, Nf6 or d5 is played to directly challenge this.

Yes, in some lines white has some trouble regaining the c4 pawn if it is taken. The c4 pawn in these lines is regained by first playing Qc2 and then capturing on c4. In this case black can play first a6 then hit the recapturing Queen with b5 and fianchetto its own bishop on b7 with tempo. It all looks fine for black but white has serious underlying threats. I was able to hold such a position but with no small difficulty.

Can you still play the Catalan setup after the QGA?

I have some doubts even about the Nimzo-Indian system. Since black fails to play d5 in some cases white manages to push the d-pawn and maintain it on d5. The position though in terms of theory equal can get somewhat uncomfortable.

Catalan isn't dangerous..., and certainly not the MOST dangerous opening, it isn't even one of the more challenging variations for black to face after 1.d4.  

Do you mean 1. d4 d5 2. c4 dxc4? I consider this line even more risky than the Catalan itself...

Then I must be doing something terribly wrong since all my big and painful losses came from the Catalan.

The QGA is not a "line", it's an independant opening by itself, and a well respected one.

Maybe a little unfashionable nowadays, but so is the French, for eg.

Well everyone has preferred positions and positions they dislike, but for me personally I have never thought the Catalan to be testing in the least...