user872: I would like to learn a good response against the Sicilian defense (in your opinion), for both blitz and slower games. 
I am looking for a more aggressive response, since it fits best my style. Currently I am trying to learn the Grand Prix attack. I have had good wins with this system but also crushing defeats, and I am wondering if it is the best system to use. I am rated around 1500.

user12533: As you wanted aggressive, the Levenfish attack can catch a lot of players out and black needs to know what he's doing to defend against it. I'll put the opening moves but leave the analysis to you :)
1 e4 c5 2 Nf3 d6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 g6 6 f4

user7002: Smith Morra gambit is even more aggressive than Grand Prix (It's a gambit!)
You'll have more decisive wins (lots of sacrifices) and more tragic losses.

user11021: I am surprised nobody mentioned the Alapin Variation yet. It is played at all levels and considered a very solid Anti-Sicilian, not without reason:

e4 c5
c3

After 2. c3 white aims for a bold center with the move d4. The benefit (for white) is that many times black is not familiar with the arising positions, which can look very different from what Sicilian players usually expect.

user307: 
I have had good wins with this system but also crushing defeats, and I
  am wondering if it is the best system to use.

You are not the only one who experienced this with the Grand Prix.
According to this: 

The Grand Prix Attack is a lazy player approach to fighting the
  Sicilian Defense. It can lead to early knockouts as in Carlson's (sic)
  victory against Veselin Topalov, or getting busted in ten moves, as in
  Carlsen vs Anand

The games in question are:
Carlsen - Anand (Blindfold at Monaco)
[FEN ""] 
1. e4 c5 2. Nc3 d6 3. f4 g6 4. Nf3 Bg7 5. Bb5+ Bd7 6. Bc4 Nc6
7. O-O Na5 8. d3 Nxc4 9. dxc4 Bxc3 10. bxc3 Bc6 11. e5 Qc7
12. Qd3 f5 13. Ng5 h6 14. Qh3 dxe5 15. Be3 e4 16. Ne6 Qc8
17. Nxc5 Nf6 18. Bd4 Kf7 19. Rae1 a5 20. Re2 b5 21. Nb3 bxc4
22. Nd2 Bd5 23. Rfe1 Qd8 24. Qh4 e6 25. Rb1 Qe7 26. Nxc4 Nd7
27. Qxe7+ Kxe7 28. Ne3 Rhb8 29. Rxb8 Rxb8 30. c4 Bc6 31. Rd2
e5 32. Bxe5 Nxe5 33. fxe5 f4 34. Nd5+ Bxd5 35. Rxd5 Rb1+
36. Kf2 Rb2 37. Rd4 Rxc2+ 38. Kf1 f3 39. gxf3 exf3 40. Rd6 g5
41. Rxh6 Rxa2 42. h3 a4 43. Rf6 Ra1+ 44. Kf2 a3 45. Ra6 a2 0-1

I'm guessing that Carlsen is busted due to tripled pawns.
Carlsen - Topalov (Rapid at Monaco)
[FEN ""]
1. e4 c5 2. Nc3 d6 3. f4 Nc6 4. Nf3 g6 5. Bb5 Bd7 6. O-O a6
7. Bxc6 Bxc6 8. d3 Bg7 9. Qe1 Qd7 10. a4 f5 11. Nd5 fxe4
12. dxe4 Rb8 13. Ng5 Bxd5 14. exd5 Qf5 15. Ne6 Bf6 16. Qe2 h5
17. Ra3 Nh6 18. Rg3 Kd7 19. Rg5 Bxg5 20. fxg5 Qxd5 21. Nf4
Qd4+ 22. Be3 Qe4 23. gxh6 Rxh6 24. Nd5 Rhh8 25. Qd2 Rhf8
26. Re1 Rf5 27. Nb6+ Kc6 28. Bxc5 Rbf8 29. Bd4 1-0


user924: The Open Sicilian (2.Nf3 and 3.d4) is the most aggressive way to attack the Sicilian, as white immediately opens up the position for all his pieces and gets a nice knight on d4. Black has trumps, but first he lags in development and his advantages typically only start counting if he survives white's attack. It is also the best reply, or at least it has consistently been the most popular reply to the Sicilian on higher levels since forever.
So I think that it's really the only correct answer to your question, which asks for the most aggressive and best response to the Sicilian.
That said, it is not the easiest. Black has like ten different ways to react to the Open Sicilian, and although white gets great chances of an attack in almost all of them, they are all different and often black knows them better than white. Spending loads of time learning everything is probably not the most efficient way to improve your chess right now. That is a general thing with chess, and life -- ambitious, good and easy, choose two.
You say "I have had good wins with this system but also crushing defeats". That is good, that is exactly what you expect from aggressive openings, it seems to me.
You also say "I am looking for a more aggressive response, since it fits best my style." That is less good. At top level, having a style at all is a sign of weakness, as it means you are biased. But you're 1500, you don't have a style. You simply have some parts of the game that you are bad at and some parts that you are less bad at. You should be busy identifying things you're bad at and working on them, not worrying about your style.
And, last, losing some games is the worst reason to switch openings! You'll just lose some more games in the next variation you chose, switch again... The Grand Prix is a decent variation, it didn't cause your losses. You made mistakes later on. Find out where, and do better next time.
Understand what happened more generally -- what plan did you choose, how did you expect it to work, why didn't it, is that a problem in general or is it just because of the moves he happened to play this time, in what kind of situations would it work? What other plans are there? Find example games. Regularly switch variations inside the Grand Prix attack, that way you'll see more typical positions and if your opponent does something unexpected, you'll have broader experience to draw from.
Become an expert in your Grand Prix.

872: <p>I would like to learn a good response against the Sicilian defense (in your opinion), for both blitz and slower games. </p>

<p>I am looking for a more aggressive response, since it fits best my style. Currently I am trying to learn the Grand Prix attack. I have had good wins with this system but also crushing defeats, and I am wondering if it is the best system to use. I am rated around 1500.</p>
 12533: <p>As you wanted aggressive, the Levenfish attack can catch a lot of players out and black needs to know what he's doing to defend against it. I'll put the opening moves but leave the analysis to you :)</p>

<p>1 e4 c5 2 Nf3 d6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 g6 6 f4</p>
 7002: <p>Smith Morra gambit is even more aggressive than Grand Prix (It's a gambit!)</p>

<p>You'll have more decisive wins (lots of sacrifices) and more tragic losses.</p>
 11021: <p>I am surprised nobody mentioned the <a href="https://en.wikipedia.org/wiki/Sicilian_Defence,_Alapin_Variation" rel="nofollow noreferrer">Alapin Variation</a> yet. It is played at all levels and considered a very solid Anti-Sicilian, not without reason:</p>

<ol>
<li>e4 c5</li>
<li>c3</li>
</ol>

<p>After 2. c3 white aims for a bold center with the move d4. The benefit (for white) is that many times black is not familiar with the arising positions, which can look very different from what Sicilian players usually expect.</p>
 307: <blockquote>
  <p>I have had good wins with this system but also crushing defeats, and I
  am wondering if it is the best system to use.</p>
</blockquote>

<p>You are not the only one who experienced this with the Grand Prix.</p>

<p>According to <a href="http://www.chessbase.com/newsdetail.asp?newsid=7110" rel="nofollow noreferrer">this</a>: </p>

<blockquote>
  <p>The Grand Prix Attack is a lazy player approach to fighting the
  Sicilian Defense. It can lead to early knockouts as in Carlson's (sic)
  victory against Veselin Topalov, or getting busted in ten moves, as in
  Carlsen vs Anand</p>
</blockquote>

<p>The games in question are:</p>

<p>Carlsen - Anand (Blindfold at Monaco)</p>

<pre><code>[FEN ""] 
1. e4 c5 2. Nc3 d6 3. f4 g6 4. Nf3 Bg7 5. Bb5+ Bd7 6. Bc4 Nc6
7. O-O Na5 8. d3 Nxc4 9. dxc4 Bxc3 10. bxc3 Bc6 11. e5 Qc7
12. Qd3 f5 13. Ng5 h6 14. Qh3 dxe5 15. Be3 e4 16. Ne6 Qc8
17. Nxc5 Nf6 18. Bd4 Kf7 19. Rae1 a5 20. Re2 b5 21. Nb3 bxc4
22. Nd2 Bd5 23. Rfe1 Qd8 24. Qh4 e6 25. Rb1 Qe7 26. Nxc4 Nd7
27. Qxe7+ Kxe7 28. Ne3 Rhb8 29. Rxb8 Rxb8 30. c4 Bc6 31. Rd2
e5 32. Bxe5 Nxe5 33. fxe5 f4 34. Nd5+ Bxd5 35. Rxd5 Rb1+
36. Kf2 Rb2 37. Rd4 Rxc2+ 38. Kf1 f3 39. gxf3 exf3 40. Rd6 g5
41. Rxh6 Rxa2 42. h3 a4 43. Rf6 Ra1+ 44. Kf2 a3 45. Ra6 a2 0-1
</code></pre>

<p>I'm guessing that Carlsen is busted due to tripled pawns.</p>

<p>Carlsen - Topalov (Rapid at Monaco)</p>

<pre><code>[FEN ""]
1. e4 c5 2. Nc3 d6 3. f4 Nc6 4. Nf3 g6 5. Bb5 Bd7 6. O-O a6
7. Bxc6 Bxc6 8. d3 Bg7 9. Qe1 Qd7 10. a4 f5 11. Nd5 fxe4
12. dxe4 Rb8 13. Ng5 Bxd5 14. exd5 Qf5 15. Ne6 Bf6 16. Qe2 h5
17. Ra3 Nh6 18. Rg3 Kd7 19. Rg5 Bxg5 20. fxg5 Qxd5 21. Nf4
Qd4+ 22. Be3 Qe4 23. gxh6 Rxh6 24. Nd5 Rhh8 25. Qd2 Rhf8
26. Re1 Rf5 27. Nb6+ Kc6 28. Bxc5 Rbf8 29. Bd4 1-0
</code></pre>
 924: <p>The Open Sicilian (2.Nf3 and 3.d4) is the most aggressive way to attack the Sicilian, as white immediately opens up the position for all his pieces and gets a nice knight on d4. Black has trumps, but first he lags in development and his advantages typically only start counting if he survives white's attack. It is also the best reply, or at least it has consistently been the most popular reply to the Sicilian on higher levels since forever.</p>

<p>So I think that it's really the only correct answer to your question, which asks for the most aggressive and best response to the Sicilian.</p>

<p>That said, it is not the easiest. Black has like ten different ways to react to the Open Sicilian, and although white gets great chances of an attack in almost all of them, they are all different and often black knows them better than white. Spending loads of time learning everything is probably not the most efficient way to improve your chess right now. That is a general thing with chess, and life -- ambitious, good and easy, choose two.</p>

<p>You say "I have had good wins with this system but also crushing defeats". That is good, that is exactly what you expect from aggressive openings, it seems to me.</p>

<p>You also say "I am looking for a more aggressive response, since it fits best my style." That is less good. At top level, having a style at all is a sign of weakness, as it means you are biased. But you're 1500, you don't have a style. You simply have some parts of the game that you are bad at and some parts that you are less bad at. You should be busy identifying things you're bad at and working on them, not worrying about your style.</p>

<p>And, last, losing some games is the worst reason to switch openings! You'll just lose some more games in the next variation you chose, switch again... The Grand Prix is a decent variation, it didn't cause your losses. You made mistakes later on. <em>Find out where</em>, and do better next time.</p>

<p>Understand what happened more generally -- what plan did you choose, how did you expect it to work, why didn't it, is that a problem in general or is it just because of the moves he happened to play this time, in what kind of situations would it work? What other plans are there? Find example games. Regularly switch variations inside the Grand Prix attack, that way you'll see more typical positions and if your opponent does something unexpected, you'll have broader experience to draw from.</p>

<p>Become an expert in your Grand Prix.</p>
