What's the most offensive opening for Black? I'm black a lot and I really like being offensive, but turning it around on white can be really hard, so what's a good opening?

Please don't say fools mate or scholar's mate etc.
This is an incredibly general question, because obviously white is allowed to make some moves as well … there is no one fits all aggressive defence. 
But to give an equally general answer: If you like to play aggressively playing gambits makes a lot of sense. So everything with "gambit" in its name is a valid choice. 

I'll give a few examples and will assume that you didn't specify whites opening because all your opponents play 1.e4. ;-)

This would be the Latvian Gambit against the Italian Game:
http://en.wikipedia.org/wiki/Latvian_Gambit

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

1.e4 e5 2.Nf3 f5


This is the Schliemann-Jaenisch-Gambit against the Ruy Lopez:

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

1.e4 e5 2.Nf3 Nc6 3.Bb5 f5


Generally the idea of these gambits is to speed up the development of your pieces at the cost of a pawn and to gain greater central control by diverting your opponents central pawns. 
If you play well, central control and active pieces will lead to an attack. 
If you play badly, you're just down a pawn ...