What is a good, aggressive response against the Sicilian Defense? 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.
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.