The Slav (1 d4 d5 2 c4 c6) is very solid and is a tough nut to crack. Many great players have played it, including Capablanca. The only drawback is that if White plays the Exchange Variation ( 3 cd cd), he can make Black's winning chances quite slim if he avoids complications. As such, the Slav is not an appropriate opening if as Black you are playing for a win.
What to play if you are playing for a win "at all costs?" The Tchigorin Defense is what I would recommend (1 d4 d5 c4 Nc6). Black often castles queenside resulting in a very exciting game.