How About the BENKO GAMBIT!!
My friend just started to play d4 on me and like you i was not very used to 1. d4. Then I learned that the benko gambit is one of the most well respected gambits and it is for aggresive players.
This is the Ideal setup
This is all i know about the benko gambit and I have beaten my friend a few times with it
i 2nd the benko. it's fun to play for sure.
It depends how much time you have on your hands. If you have a lot of to learn a very solid defence, go with the Nimzo-indian complex, which I don't particularly like because there is just too much theory behind it and instead of learning one defence, you have to learn three, the standard Nimzo, the QID or the Bogo and the Catalan. Granted, they're very solid but very demanding.
If you have some time, learn the Gruenfeld. It's very easy to rip apart medium class players with it, once you get a good enough feel for the opening and what your ideas are. Again, there is a massive amount of theory to learn, but considerable less than with the Nimzo-indian.
If you don't like that either, go with the KID. It's also very theoretical but there are considerable fewer lines and less memorizing involved, since in many lines, you'll be focusing on concepts and ideas instead of move orders.
As for other types of defences. Yes, the Slav is very good but again you have a lot of theory to learn and there are quite a few annoying lines, like the exchange and a few others that can ruin your fun against a mediocre player.
The old QGD is good but you rarely have winning chances.
The only part of the QG's I like is the QGA, where it's very hard for White to go for a sterile drawing line, even in the dxc5 line, where there is an early Queen exchange, you still have good chances to win with Black.
As for the other recommendations given here. The Dutch will bring you pain and pleasure, depending on the kind of opponent you face. On the whole, I never trusted that opening, you just risk too much for too little. Btw, playing 1. Nf3 f5 can lead to a very interesting gambit with 2. e4.
The Budapest is dodgy. Again, it has some surprise value but against a decent opponent you'll be struggling from the start.
The Benoni is something else. I'd call it extreme sport in chess. It can work but boy can it fail. One small mistake and White will pounce on you with his or hers central pawns and you'll be biting dust. Again, the opening is pretty sound but you need a lot of patience to make it work.
As for the Benko, I use it regularly in blitz games. It's very easy to use in a fast game, since you don't have to react to White's plan all that much and initiative is all that matters in blitz. As for using it in a normal game, I'd think twice about it, but I'd certainly trust it more than the Dutch or even the Benoni.