Gothic 2, no question about it. It actually is a hack-and-slash game with a deep storyline, exactly what you're looking for! It's seriously an awesome game. Best I've played lately apart from Planetside, but that's a pretty different kind of game.
Sacred isn't as good as Diablo 2 at making combat interesting in my opinion. After a while I was reduced to running to the next story-based encounter and stopping ever 30 seconds or so to use "Fist of God" on the horde of monsters that had built up behind me. Haven't finished it yet. Doubt I ever will.
