Demon's Souls for me, but I should note have not played Dark Souls II yet.
Mainly because in this current climate of gaming it really stuck out as a throwback in tone. At first it was punishingly brutally difficult, then you realize have to rewire brain back to old days of memorizing patterns, being patient and picking moments.
I don't think any sequel will recapture fully the original's appeal as it came out of left field as such a refreshing game.