Fried Liver Attack is very aggressive.
What are the most aggressive openings ever?
I think that as white, there is no question, that hands down, #1 is the King's Gambit!!! (My fav) Otherwise the dutch gambit is quite agressive as well (and exciting). Then there's the nutty ones like the Halloween Gambit - hard to get much more agressive than that! A simple look at Morphy's openings (renowned for their agressive and fast nature) will reveal a gold mine of these!
As black, I would lean towards the center counter game (1.e4 d5) as it often brings out the queen early and makes more early tension and pressure in the game. Almost as agressive if not more at times, is the relatively obscure 'Bird's defence to the Ruy Lopez' (1.e4 e5 2. Nf3 Nc6 3. Bb5 Nd4!?) which has some sharp and interesting lines, including the flight of white's light bishop...
Just off the top of my head...
I forgot about the bird's opening!!!
Bird's opening isn't aggressive. From my experience against it, it's one of the least aggressive and easiest for black to equalize against.

I used to play the Bird a lot. Every game used to be the Bird. And from what I gathered, it was not too aggressive. The aforementioned King's Gambit is quite aggressive, along with, in my opinion, the Ruy Lopez. The Grob, 1. g4, I can't help but think is aggressive in nature, albeit it gives Black opportunities for a counterattack due to the openness of White's king (that's why you castle Queenside when you can in the Grob!).

http://www.rci.rutgers.edu/~goeller/urusov/perreux/lolli_attack.html
The Lolli attack reccomends 6. d4 instead of the usual line, 6. Nxf7.

Fried Liver Attack.
Followed by the Lolli Attack.
Any "attack" that relies on your opponent making a mistake isn't worth much discussion. By that criteria, Fool's Mate is the most aggressive opening that exists.
As far as openings that consider best-response... hmm, there are a lot of crazy candidates. There's nothing saying that YOUR moves have to be good, so some of those early Nxf7 variations have to qualify, many of those are well beyond sound. Come to think of it, there are a lot of variations in the King's Gambit of questionable soundness.
If you want something relatively *sound* as well -- or at least, "unrefuted" -- there are still a lot of choices. Najdorf Poison Pawn, Danish Gambit, Blackmar-Diemer Gambit, Latvian Gambit are all legitimate candidates. But I'd have to vote for the Traxler Counterattack (1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 Bc5!?). That thing is crazy.

Traxler Variation in the Two Knights Defence, Belgrade Gambit in the Four Knights Game, From Gambit in Bird's Opening, Frankenstein-Dracula variation in the Vienna Game, Dragon Sicilian, and the Double Muzio Gambit in the King's Gambit to name a few.
Kings pawn game, Fried Liver is the most aggressive. Ive been using most of my life and that is why. The Halloween Gambit is a really aggressive line in the 4 knights opening which is what I usually aim for if they move knc6 instead of knf6. And I have to disagree with some others in saying that Bird's Opening is rather aggressive depending on the line look at the From Gambit games that have been played and has been used by many GM's to varying degree. Of course it is a gambit and must be accepted to go into those lines. But by far the Fried Liver is the most aggressive I know of. I learned it getting stomped by e-games chess engine on the hardest level until I got frustrated and started sacrificing my Bishop Bc4xf7 on the fourth move, then I beat it in 7, 11, 14, and 21 moves, but i dont recommend the sac against human players or rated games, that was more of a programming error on their part. But I can't think of a more aggressive opening than that. ;)
What are the most aggressive openings ever?