Sicilian defense, Sheveningen with Najdorf move order.

6.Bc4 is weak in the Najdorf. 6.Be3 is probably the strongest but my choice is 6.Be2, which I think is theoretically better. And against the Scheveningen (6...e6) 7.Be3 is imo the best continuation. So when will you apply the Dragodorf (...g6) against what I think is the best WT setup against the Najdorf/Scheveningen structure?

When I have faced this, my favorite line has been

1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nc6 5.Nc3 a6 6.Bc4 e6 7.Bb3 followed by 8.f4 and 9.f5 putting black under immense pressure early, and it has been played by Fischer if I remember correctly.

Yeah, Fischer liked that one, and I do too. Yet you shouldn't forget about the 7. g4 in the Be3 Scheveningen. That's a Delayed Keres Attack, and then we can talk about the Perenyi Gambit too! And THAT is a hundred percent sea of sharpness and tactical ludacris! Also, the Bg5 variation classifies as the Najdorf Old Main Line (White plays f4 next), and when Bc4 is played alongside Be3, assuming that Black plays the Scheveningen-more-or-less-thematic Nc6, we arrive at the Velimirovic Attack! And that's another one you wouldn't really like to mess with (if you were Black, of course)!

Maybe I'm missing something here, but why does a6 prevent g4? I don't see why White can't just wait another move to play it, he's not blocking the pawn or anything.

6.Bc4 against the Najdorf, the Fischer-Sozin Attack, is pretty neat. I used to play it a lot, and enjoyed great success at the club level with it (until people got too used to it, that is!). I don't think that 6.Bc4 is weak at all.

No, you don't have time for 8.f4 and 9.f5. The king is still in the center and 7...b5 is faster to attack the queenside. If you want to castle kingside, there is ...Bb7 following ...b5.

Can anyone familiar with this variation tell me whether black castles kingside when white castles queenside with an English attack pawnstorm? Seems to me that in many of the games in the database show black's king remaining in the center, or in some situations even castling queenside as well despite not always having pawn coverage. 

Chess is "theoretically" a draw. Any theoretically best defense is a defense that will lead to 100% draw. Often we are not looking for a draw but we are looking for positions that will give us higher probability of winning (more attacking chance, more initiative, lots of space, etc.) which I consider a "strong" position. This is of course subjective because it depends on the level of play.

To make things less subjective, we may want to define an "average level of play", "average type of game", etc.

It's like 1.e4 and 1.d4. I consider 1.e4 as "stronger" than 1.d4 but 1.d4 is "theoretically better".

That kind of statement/terminology may not be accepted universally. But that's my point.

First, you have to remember the concept of tempo. Many Sicilian games are opposite-side of attack, or race of attack. Just like chess in general, g4 on 6th move will be different than g4 on 7th or 8th move. Here, the line (and game) will completely be different.

In Scheveningen, e6 is done on 5th move. This e6-pawn blocks the c8-bishop's control on g4-square. This allows White to do the scarry Keres Attack (6.g4 often folowed by h4) when Black is not yet ready to defend or counter-attack. With 5...a6 White cannot do 6.g4 before adding more control on g4-square with 6.f3 or 6.Be2 (6.h3 is weak because the pawn is needed for the attack). But then g4 will lost its strength because BL is better prepared than before.

Yes I love to play 6...Ng4. It is imo one of the strongest line in the Najdorf. Just like the Najdorf in general, BL has to memorize many variations. Especially the 6...Ng4, I can play many variations from this position like machine. This is where memorizing lines becomes crucial, especially when the best move is not natural.

Personally I prefer 6.Be2 than 6.f3. With f3 you have to castle queenside if you want to do the (Perenyi) attack, and BL get his counter attack relatively quickly. With 6.Be2 you can do kingside attack with 0-0 or 0-0-0, you can attack in the center (with f4/g4) or in the wing (with g4/h4) and BL has to defend and get the counter attack only after White fails his attack. I like this feeling of controlling the game.

I get this up until the last sentence. Why is g4 inherently weaker once a6 has been played? Surely if black plays e6 anyway the move is still good.

Actually there's nothing wrong. The term "weaker" is infact subjective. We are here comparing what is called Keres Attack with other kinds of English Attack (e.g. Perenyi Attack). If you have ever played the BL side with those attacks, you may hate the Keres Attack, which is more dangerous because your pieces are not yet well developped. But again, you may find that 7.g4 or 8.g4 or 9.g4 is still playable, and is also a common line.

Also, if you choose the Amsterdam Variation of the Najdorf (6. f4), and your opponent plays 6... e6 instead of 6... e5, then you can transpose to the Tal Variation!

Yeah, I had to double check it to make sure. When I cracked open my copy of MCO I almost facepalmed because I was 99% sure that it /was/ the Fischer-Sozin, since that's the way my mentor presented it to me. Kind of confusing, because they're somewhat similar!