Descent and Descent 2 are my favorites... I probably like D2 a bit better, since it is more varied.
I didn't like D3 or its expansion very much. I felt they deviated a bit too far from the formula of the originals. I'd've been happy with another game identical to the original two, and that was rather what I was expecting, to be honest. So I was disappointed big time.

I keep D3 around because I might get around to trying out the multiplayer someday (since Nuke's Descent 2 multiplayer isn't seeing much action...).
Also... shouldn't this be in the Gaming Discussion forum?