6.Bg5 is the best move, and it is the reason that the Classical is currently unpopular for Black at top level; White is supposed to have better chances in several lines, I don't know which.
If there were a rule saying that White could only play Bc4, Be2 or f3, then I think the Classical would be the most popular Sicilian :-)
In my games on chess.com and in viewing a lot of Sicilian games in general, I haven't seen the Richter-Rauzer (1.e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 Nc6 6. Bg5 or 1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 d6 6. Bg5) appear with as much frequency as it seems to in databases. Shirov, Korchnoi, and Kramnik are all big names that have had good success with the Black side of this line. Given White can play 6. Bc4 and go into the Fischer-Sozin, 6. f3 and head towards Dragon lines, or 6. Be2 and hit either the Classical or Boleslavsky.
Some argue that the Nb3 lines (6. ... e6 7. Qd2 Be7 8. O-O-O O-O 9. Nb3, for example) stifle much of Black's Queenside counterplay and don't allow enough opportunity, unlike the Najdorf or even the Dragon, in some opinions.
What do you guys think about the opening, either as White or Black? As White, does 6. Bc4, 6. f3, or 6. Be2 hold more potential? As Black, do you feel you have good chances?