Ravenloft seems pretty good to me. It's mostly an update of the original adventure, with more meat (IMO).
Demonweb Pits has a plot that seems more in line with devils than demons. It really suffers from the railroad syndrome - your PCs have to follow the plot, no matter how unlikely it seems. There are plenty of good encounters in it, but overall? Not sure. It's an original adventure. I really like how it deals with the Demonweb, though.
Haven't got Undermountain yet. I have no problem with the way it's set up, though. I'm not expecting every encounter area to be detailed.
Cheers!