I always enjoyed the U-series of modules. I've used U1 to launch a number of games, either as part of the series or a one-off. This contains some spoilers.
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U1 The Sinister Secret of Saltmarsh
by Dave J. Browne with Don Turnbull
32 pages (one blank, one full-color illustration, one with PC handouts)
U1 The Sinister Secret of Saltmarsh is an adventure for 5-10 level 1-3 characters for AD&D. The setup is pretty typical - haunted house on the hill, there are monsters in it, the town will pay you cash money to go deal with it.
What I really like about U1 is its multi-part nature. If the PCs find all the threads, they get to explore a haunted house, find a sinister secret about it, and then pursue that secret into a ship-borne adventure. You go from gathering information in town to assaulting a ship in a few little hops. There is information to be had in town, too - with the option of just skipping it and losing out on what can be very helpful or accidentally misinformative.
There are some nice touches in the adventure, too - a plant by a local in cahoots with the bad guys. It's a fairly obvious one, I've never had anyone fall for it. But I have had a group make the leap to "hey, not everyone in town is on the same side!" which is more likely the actual point. The bad guys make mistakes; this is one of them and the module points that out. There is a clear link to U2 placed in the adventure, and it's another thing that needs to be ascribed to "people messed up" or "trust is an issue with the bad guys." I guess the guys in U2 dealing with the guys in U1 sure don't trust them without putting their own men on the scene. Seems fair.
The are is good, too, and depicts a few of the more interesting rooms, the house itself, the ship's nasty-looking crew, etc.
One thing I especially liked is that the "special notes" for the DM are really helpful. It covers a lot of potential PC action and gave guidelines for handling them, everything from resisting excise taxes to trying to fool their way about ship to using stealth by fairly non-stealthy PCs. As a young GM, that was very useful to me - instead of yes-ing or no-ing things out of hand, I learned how to assign a chance and let the dice dictate it. I recycled some of the bits of this adventure and those rulings in later adventures and later games. It even had specific notes about the characters losing out if the players aren't be attentive when it matters. That suits how I play now - character skill and player skill both matter.
The adventure also carefully details the ship - I've never lacked for a needed detail when running it, and it's informed how I've statted up details for ships in other games. There is a lot of text to read, though, but it's uniformly good and useful stuff. Also, there is a roster of significant foes (or at least, an appendix of stats) and the same details are where they are needed, with the encounters. It makes it easy to run and easy to run after a party comes, leaves, and the NPCs move around.
The place is a little heavy on magic items, in my opinion, but you'd have to play very well and make a lot of good decisions and good rolls to find and get them all.
I've run U1 so many times it's hard to keep them all straight.
I remember running it for my three-player game in Junior High School. They cleaned out the haunted house and the ship, and I remember they kept a lot of the silk and brandy to have made into fine clothes and to stock their house. That same trio cleaned out a mansion in a Dragon magazine fold-out adventure and moved in. They'd eventually go on to U2, as well, but never made it to the third module.
I ran my cousin through these with a solo thief. He made it through U1, U2, and U3 as well. He eventually assassinated the sahuagin chief in a nasty backstab-started fight and escaped.
In GURPS, I used the haunted house to start one campaign. A couple players tried it, and lost a few characters in a couple of delves. The first group never found the secret of the house, and just left. The second group just got killed off by some terrible tactical choices. Memorably one guy was killed with the ol' stirge to the eye. This is an adventure very well suited to GURPS, in terms of the types and numbers of foes encountered.
I do remember more than one group strongly resisting paying excise taxes on recovered goods. Just goes to show that taxing PCs is a good way to drive people Chaotic, even if it totally and logically fits the situation.
Overall: This is one of my favorite modules, and even if you don't like to run modules, it can give you a lot of useful rulings, details, set-pieces, maps and ideas for elsewhere. The is one of the better AD&D modules.