It's a tie between White Plume Mountain way back when that was a new module and going ALL THE WAY through Return to the Tomb of Horrors when I was last stationed in Japan a few years ago.
White Plume Mountain is mostly memorable because we spent most of the adventure against a fellow player. See we were going thru the water filled corridors at the beginning and got ambushed by ghouls. The Dwarf in our party (the one who tried to kill us the rest of the adventure) was in the lead (so we could fire over his head if attacked) and he got hit by almost all the ghouls in the first round when they came up out of the water and failed all but one save against the paralyzation. The way our DM played multiple effects like that was that the durations stacked. So he was gonna be paralyzed for almost half an hour. But we didn't know this as characters. We were very good about not playing with out-of-character knowledge so all we knew was we just saw him get hit 8 times in an ambush and drop- we assumed he was dead. After we finished off the ghouls we came back and, still assuming he was dead, did what all good adventurers would do- we looted his "corpse"!
Needless to say the Dwarf's player was quite upset by this.
So we finish that part of the dungeon and start back for the other parts. When we come back to where we left our "dead" companion, he's not there! We had to assume he'd become a ghoul, AND we figured he was hunting us down somewhere in the dungeon- which turned out to be true. When his paralysis had worn off, he had vowed to kill us both and get his stuff back. We spent the rest of the dungeon not only fighting the local critters but also playing cat and mouse with the Dwarf. It eventually culminated in an ambush we set in the room beyond the disk bridge after we had defeated the vampire guardian there. We set multiple traps along the approach corrider and were set to harrass him with bolts and arrows as he attempted to cross the disk bridge over the geysers. It turned into an anti-climax though, as he got knocked off the second disk when one of the geysers blew and he died in the boiling mud.
Great fun though, for everyone !
Return to the Tomb of Horrors was fun as a long excercise in perserverance and greed.
It turns out I had an old character which had partially been thru the Tomb of Horrors, but had never finished it because too many party members got killed. The DM let me play this character since he knew the original dungeon wasn't the main challenge for us. Plus the city and second dungeon would be all new anyway. So since we were high enough level to not be very challenged by the original dungeon (being VERY cautious, smart and with a partial guide), we spent most of our time there scrapping every last bit of wealth we could out of it. It drove the DM crazy but between TWO portable holes, the Shrink Item spell and just being very determined/inventive we managed to get almost every last copper out of there. (We even came back for the mithral and adamantite doors after the whole things was finished and Acererack's magical protections were gone!!) Then we just had to hang on and fight our way thru the rest of it! A very challenging adventure and quite alot of fun.
