1, while 2 was more fun the first time 'round, and more challenging, the third and fourth times through, DMII seemed to have almost no plot. Sideplots, yes, but no real plot other than "Kill the big evil thing" and then the ending really bothered me! I was angry for weeks at that ending!
DM, on the otherhand, actually made me think several times, and caused me to scratch my head a few times. Plus, I gain great satisfaction from completing DM with a 1pc party. You simply cannot do that in DMII