Similar things for me:
I6 : Ravenloft
A4 : Can't remember the exact name, it's the last of the slaver series, where the players have to escape from the 'Earth Dragon' - I actually like the whole series, even though it's a bit of a railroad fest.
The U series; mostly U1, as it's a superb module for new players; U2 does have the flaw (as pointed out above) that it can be finished ridiculously quickly (and was, when I DM'd it many years ago), and U3 was just brutal at the suggested levels - especially as there wasn't that much xp in U1, and ostensibly none in U2 if it went right
Recently though my love for d2 has known no limit for the Worlds Largest Dungeon. I know not everyone will like it, but the way that each area is set-up, with it's factions and plots, reminds me of The Vault of the Drow - it's just a great adventure for those who want to pick it up and play, or whether you want to customise it to your every whim (which is nice).
Jim.