Ah... B1... it's the stem cell from which all other adventures mutate from.

I had always hoped that Rogahn and Zelligar would return, turn out to be uber bad guys and try to take back their dungeon... or more levels would open up or there would be an adventure where they are good guys, but you have to go rescue them.  etc..

Great stuff.  The first time we saw Magic Mouths in the hall way we all crapped our pants and knew.. This game was going to be WAY better than Stratego!

-Jon

I'd have to say X2 Castle Amber.

Castle Amber was great and was the only D&D module I played until the Companion rules came out (except for Keep on the Borderlands - of course).

X2 is also part of the "geekiest" Dungeons & Dragons incident of the my life.  I've been holding back this confession for a long time and it is time to actually let it out. . .

. . . Back in 6th grade my friends and I used to play  D&D on our recess period.  Our teacher (I forget his name) took notice and between me, my friends and him it was suggested that we play the game in front of the class and he would give us extra credit. . .

So. . . I was the DM and Greg, Steve, Wade & Tom were the players.  We proceeded to play D&D in while the rest of the class was gathered around. . . and our teacher let us play for over 2 hours!  As you can imagine. . . not one person was interested (if any of them had been, they'd have been playing with us to begin with).  Absolute boredom and blank stares surrounded us. . . I don't think any of us cared at first, but towards the end it did start to get a little. . .  embarrassing  

But, what the hell?  We got to play D&D for the afternoon and he gave us all an A+ in extra credit.

At least I've grown up to be  totally cool, though.  Right?  I mean, between the Spider-man movies and Lord of the Rings my status has gone way up - without me, how would any of my co-workers know the acutal origin and complete history of Spidey & the Green Goblin, not the mention the deeper meaning and history behind the each and every scene in LOTR.

Like I said. . . totally cool.
                         

My favorite single module is G3, though I love the S4-WG4 one-two punch

btb, I taught kids to play D&D in after-school "Enrichment Program" classes:  I had a basic/intro class, a DM class, and an advanced class.  I'm sure that I've still got the syllabi around somewhere....  Come to think of it, they were my first professional gaming efforts, since I got paid about $90 per class IIRC!

Favorite module on paper or based on personal experience is a big difference...and I'm not sure I could pick either way.


Personally: I12 Egg was a fantastic epic
L2 Assassin's totally changed my concept of what an adventure could be
DDA1-2 Thyatis was incredibly well written.  Also, one of my best DMing efforts ever.

On paper:
UK2-3 Alderweg have to get major props as my Most Underrated pick
I6 is in no danger of underrating, but deserves its rep
I3-5 Desert was a tremendous series, although we had no idea how to believably incorporate it into our generic campaign world and even less idea how to run an 'Arabian' campaign.
B4 had some phenomenal semi-developed ideas (particularly the world beyond the bottom of the temple) that I could have gone wild with, if we'd have played it when I was older.

You didn't have S4 up there, so I had to go with my #2 - Queen of the Spiders.  

Actually, it's a tossup between S4 and B2 (the first one I ever played)

S1 is a great one, but a killer module.  You really have to be on your toes for that one.  And the Desert of Desolation series is the type that really sucks you in.  The kind of modules that you wish would never end.

Ecthelion

If I could chip in with my $0.02... I'd have to say that N1: Cult of the Reptile God deserves a nod for one of the best "beginner" modules out there.

I cut my teeth on B2 and X1, but got to this one years later. Wow. Wish I had started with N1 instead. It's seriously underrated, along with most of the UK modules (esp. UK2-3 and U1-3) and the S4-WG4 duo.

Here's why I voted for B10 …

It's a full campaign waiting to blossom.  Everything you need to encourage your players to graduate from dungeon crawls, and into the world at large, is right here.  You get a compelling setting, the borderlands of Karameikos, where the verge of human civilization meets with tribes of hostile humanoids, all under the shadow of a lost ancient culture.  And unlike GDQ and T1-4, the plot is moved forward with time and intensity.  The environment is not reactive, it is compelling the characters to act to the best of their ability.

It is one of the few TSR settings I can think of that presents the adventurers' "base camp" as a place where friends are made, loyalties bound, and true incentive is given to the rest of the adventure.  It's not just a place to stock up on supplies and slog away to the dungeon again.

[spoilers]

I changed the format of the module a little, to introduce characters.  When my players first came to Sukiskyn, they were uncomfortable -- this was clearly a role-playing only site, and they had no experience with such things.  So they were relieved when the goblin tribes attacked and had to be repelled.  Then they went into the wilderness, chasing after the goblins.  When they got back, and found out that all the people they had known had been taken hostage, they took it personally!  It was the first time my players expressed real emotion for imaginary characters, or displayed any incentive to play beyond "level, kill things, get the treasure."

[/spoilers]

Adding to the mix:  Interesting allies, compelling enemies that you would love to kill but are always one step ahead of you, intriguing side adventures that scream out for mini-adventures to explain their origin, and a dramatic chase into the lost valley itself.

This sudden advent of maturity served them well once they finally got into the war between the ancient cultures -- instead of deciding who to loot and who to kill, they were having fevered arguments with one another over which side was morally "right."  It took some doing to get them out of there.

So, this module has a complete introductory setting, a great base of operations, additional adventures in every direction, recurring enemies you love to hate, a larger town to graduate to once the players are ready to move on, and a lost valley filled with ruins and adventure hooks.  It also features dungeon delving, wilderness exploration, combat, intrigue and clue-gathering, role-playing, and a siege as well.

I love the old Gygax classics, but I think B10 is as close to a perfect module as TSR ever made.