I have all three Expedition books.
Ravenloft has more content than you'll likely ever be able to include. It's mostly well-done though. Shouldn't disappoint.
Demonweb Pits is what I'm running right now. Lancelot's comments are pretty spot on. It's a big, dumb adventure. Rather cartoonish, in fact, with characters jaunting around Graz'zts capital city like it was a big Star Wars Cantina (if you've read Downer in Dungeon magazine the last few years, that's a good reference point). And for every phantasmagoric element there's an element of demons being as petty and banal as any mortal. You actually do encounter a number of demon lords (in their lesser aspect forms) which might be cool for some, and asanine for others. For creatures of pure chaos and evil, demons sure do have a high regard for bureaucracy and following rules. Portraying them thusly is done simply as a matter of plot convenience, which is sloppy.
Undermountain is still new, but I don't like the layout. It's supposed to cover a wide range of party levels, but most things seem centered around a CR of 3 or 4.