Closed Sicilian / Second move of black

So the most played move for black is Nc6 followed by e6 and a6. Can sb give me an analysis and positional and tactical pros and cons of each of the moves? Especially Nc6 vs e6

@1
1 e4 c5 2 Nc3: 2...Nc6 > 2...d6 > 2...e6 > 2...a6 > 2...g6.
It depends what you play after 1 e4 c5 2 Nf3: 2...d6 > 2...Nc6 > 2...e6 > 2...g6 > 2...a6.
The opening may transpose if white plays 3 Nf3 or 3 Nge2.

It depends on what you play against open sicilian and what setup you opt for against the closed sicilian. Practically, all are of the same level.

2... d6 gets dominated by the Grand Prix, it's not the 2nd best move, it's quite bad actually. It basically wastes a move since you need e6 / d5 quickly against the Grand Prix, and that tempo is enough to allow white to mount a pretty brutal attack. Some people will play it because it transposes nicely for them but that doesn't make it good in this position, just convenient. Alot of people play the closed sicilian with the intention of playing the Grand Prix. As a dragon or najdorf player I would go with 2... g6 or 2... a6 before I played 2... d6 here.

2... g6 is alright, but there's this early d4 line reminiscent of the hyperaccelerated dragon where the queen comes out and it's unpleasant. You can hold but white has all the initiative and you have to play sharp and defensive for the whole opening. and you're still slightly on the back foot entering the endgame... it's not that common of a variation though, only played in 13% of games. Still compared with d6 this is much better than facing an empowered grand prix in 35% of games.

2... a6 is quite good actually, it's just a bit boring. Usually you get an expansion on the wing with b5 > Bb7 and it's kind of a boring position. But it's also fairly easy to play. If d4 is played you can choose between an improved o'kelly variation (no c3), a kan, or a najdorf. I think this move is an easy choice for a najdorf player over d6

2... Nc6 is a nice move transpositionally, probably the 2nd best move against the Closed Sicilian, but it has to face 3. Bb5+ which ... while this is objectively fine for black, white seems to have control over the game, and white scores well in practice. Nc6 also doesn't do much to challenge 3. g3 early (unlike e6), it just leads to a positional game. Nc6 does well against the Grand Prix but d5 is still slower than it would be if e6 were played earlier, and white can pin your queens knight with his kings bishop / apply pressure in some lines.

2... e6 is pretty clearly the best move. It is the strongest move against the Grand Prix and it can create chaos against 3. g3 in a way no other move does due to enabling an early d5 push. f4 / g3 are whites two most common moves...

 2... e6. 3. g3 d5 is the Korchnoi defense- 

2... e6 3. Ne2 Nc6 4. g6 is the chameleon setup, usually followed by g6 -

Grand prix you just play d5 immediately, white usually will trade off his bishop, you have no issues with early pressure on f7 and the game has already equalized. You should still know the lines a little more in depth but it's easy and fine -

The downside to e6 is you must play a french sicilian. I don't consider this a downside, but some people do... One notable improvement is, since Nc3 has been played, there's no c4 maroczy bind against the Kan. You actually are guaranteed the ability to play the Kan wing attack, probably the best line in the Kan and all you'd need to learn there. Though it should also be noted there's a tradeoff here - the chameleon can also transpose into the french sicilian, but the Taimanov (Nc6) is alot stronger against the chameleon setup than the Kan (a6) due to how it pressures d4 after g3 in the line I showed earlier. But either way works, the Taimanov is more theory, the Kan wing attack is more forcing.

In most games you won't end up transposing into a french sicilian though. For me the strength of e6 against g3 / f4 plus the fact there's no magic bullet to it for white led me to pick up it up despite being a classical sicilian player. I can't pass up the way e6 screws up all those g3 setups. But I can see an argument for playing Nc6 if your opening transposes from it. The Kan wing attack isn't really that big of a line though, I don't think it matters much, I'd rather have lethality against g3 / f4.

I always tell people, when facing 2.Nc3, it depends on your response to the Open Sicilian. White can turn around and play 3.Nf3 and 4.d4 in a heartbeat!

After 1.e4 c5 2.Nf3, Black should play:

A) 2...a6 if you are a Najdorf or Kan player (where the knight typically goes to d7) followed by 3...b5 and 4...Bb7.

B) 2...Nc6 otherwise, followed by 3.g3 g6 4.Bg2 Bg7 5.d3 d6

Najdorf and Kan players are advised against 2...Nc6 due to 3.Nf3 d6 4.d4 cxd4 5.Nxd4 Nf6 as now you are in a classical Sicilian and the knight is not where it needs to be for Najdorf and Kan players, whereas, say, a Taimanov player can easily transpose - 1.e4 c5 2.Nc3 Nc6 3.Nf3 e6 4.d4 cxd4 5.Nxd4 and now 5...a6 or the more modern 5...Qc7 and it is a direct transposition.

If you're a french sicilian player there is not a good reason to skip e6 for Nc6 or a6, e6 is just better against g3 / f4 and it avoids Bb5 entirely.