Can you find the win for black?

(1) Black is already winning the game - in the position, he/she can play literally anything that doesn't hang something and still be winning.

(2) Nxc2 is pretty easy to see - taking the knight leads to mate. Bxb3 is also winning, and so is Nxb3.

(3) Qa5 is also winning - Qa5 Kb1 (a4 Bb4, and similar situation) Bxb3 and then c2+ wins more material, and probably black will checkmate white pretty soon.

There are probably 15 different moves in the position that win.

BTW, the computer says this is a mate in 6, starting with ...Ne2+, which I didn't consider, but it doesn't matter, since this position is hopeless unless black does something like ..Qb4

Uh-huh, black is already winning, and there are many moves that win. But Ne2+ is the quickest way to victory and therefore the best move. I was curious to see if anyone could find it. Black "gives back" a piece to checkmate white. It was an interesting challenge, in my opinion

I wouldn't have found Ne2+ but wouldn't have played it even if I'd considered it. It's a totally impractical and inhuman move. Nobody would play that when Black is winning so easily by a capture on b3 or even "normal" play with an extra piece. You find that kind of move only when the position 'demands' it.

I think this puzzle would've been better if both of black's rooks were missing. 

But yeah, in a real game, if you miscalculate one of the 6 moves to mate, you lose your advantage - it makes no sense to try this, and something like ...Qa5 is better. 

"Nobody would play that" This is a serious claim and I wouldn't agree with it 100%. I think some strong players would consider it and play it as soon as they saw it leads to a forced checkmate. There are "easier" moves, we know that, but finding the best one was the challenge.

Strong players know that all wins are equal and are quite happy to find one quick and convincing way. Many weaker players will try to show off their calculation skills, do something risky and fall on their heads.

There are also people who solve puzzles which is a different activity. They would like you to state "mate in 6" which is decent. But they get irritated when you state "Can you find a win for black" when you really want them to find the quickest checkmate.

Irritated? This is a just puzzle... I did ask "What's the best move". I thought it was clear that I was really asking "what's the move (or series of moves) that ends the game the quickest?" I could have said "mate in 6" but like in this page's Tactics trainer, you don't know if it's a mate, a hanging piece, removing the defender, etc. you have to find it yourself.

In any case I apologize if I made anyone irritated. I just thought some people would find it interesting, that's why I posted it here.

Your subject title is "Can you find a win for black?" Most readers will go for that instead of the smallprint. I did. "Best move" is not really an indication of what is asked for since all puzzles want "best moves". There are several definitions for "best move" for instance "the most riskless win".

In "Tactics" there are always indications in the position whether to go for the mate, the draw or the win. This diagram would never be a tactic on chess.com because it is all too vague. Here you really have to specify mate in 6.

Of course I understand you are not trying to irritate anyone so keep on posting! Just imagine how difficult it can be for a solver to see what you expect of him.

Agree - the big title is "Can you find a win for black?" which there are about 15 solutions for. 

If you don't try to go for the mate in 6:

*You will win anyways. There's no way white can come back, as long as you don't hang everything.

If you try for the mate in 6, having calculated it:

*If you miscalculate anything, you lose most of your advantage, and may not win.

*If you do calculate it, you will win - just like in the above.

So you have nothing to gain from Ne2+. 

If you removed the rooks, like I suggested, then this puzzle would be better. But then, people would find Ne2+ because there's no other way for black to win. 

I see your point. For me the "best move" is the one that wins/ends the game in the quickest way. I thought it was implied but I see it wasn't. Sorry for not being more specific, then. I'll try to be next time, if I decide to post another puzzle.

For this puzzle, you can get rid of the 2 rooks - then the best move is the only move that wins.

Thanks for the suggestion. It's useful since I might want to post this puzzle somewhere else.

... Ne2+ aims to deflect the white queen from c1 or it puts the king on the same file as the queen making Bxb3 painful.
I know I would not have found/played it OTB. Piece up and Nxc2 puts both the a and b pawn under threat while attacking the queen. 
As a puzzle I would have found it with a big fat title clue like "Reflecting on a King attack"