Ah, this list brings back some great memories. I've played dozens of these adventures on your list, but my favorites are the ones written for the Expert Rules boxed set.
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My favorite is (and probably always be) X1: The Isle of Dread. It was an introductory module released with the Expert Rules boxed set, designed to teach the DM and players alike how to run wilderness adventures.
In the story, the party finds an old pirate's map that leads them to a tropical island in search of a treasure-filled "lost city." Once on the island, the party uncovers clues to the location of the lost city...and the horrors that destroyed it.
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x-11: Saga of the Shadow Lord is one of the lesser-known adventure modules, but is one of my favorites. It is done in two parts: in part 1, the party saves the peaceful Kingdom of Wendar by returning its stolen magical relic, and slaying the evil wizard responsible. In part 2, perhaps many years later, that same wizard has apparently returned from the dead, and threatens invasion from his blighted lands to the north. To destroy him, the party must discover the nature of his existence and locate a powerful magical artifact capable of undoing him.
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X7: The War Rafts of Kron is an aquatic-themed adventure that takes place on an immense floating city in the middle of the Sea of Dread. It introduces new monsters, spells, and rules for underwater adventuring, and gives the DM an immensely rich setting for further adventures in the area.
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X6: Quagmire! is an interesting adventure where the heroes must race against time to save a city from sinking beneath a magical swamp. Somewhere in the jungles surrounding Quagmire, a race of reptillian creatures work dark rituals that are growing and spreading the swamp, slowly consuming everything around it.
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And how can anybody forget the Desert Nomads Series (X4, X5, and X10)? This epic series of adventures takes the party deep across the burning wastes of the Sind Desert, to thwart the invasion of the Desert Nomads and slay their leader, an enigmatic villain known only as "the Master."
X4 carries the party across the sands to the Temple of Death, where the Master resides.
X5 takes the party through the Master's headquarters, and pits them up against all of his minions, traps, and secrets.
X10 takes place years after the Master's defeat...the Master has returned, and he has brought his armies to the borders of the peaceful lands. This module has a built-in mini wargame, that can almost stand alone.
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X2: Castle Amber is the standard by which all "deep roleplay" gaming sessions are measured. I can't be sure, but the setting and the characters in this story seem to be heavily based on Edgar Allen Poe's tale "The Fall of the House of Usher." Anyway. In this story, the party is trapped inside an eerie fog. In the midst of this fog is an ancient mansion with a cast of eccentric characters. To find their way out of the fog, the party must solve the mysteries of the manor and unravel the tangled story of its occupants.
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X3: Curse of Xanathon is a short detective-style adventure. The Duke has apparently fallen under a curse of insanity, and the party must discover the nature of his affliction and find a cure before it's too late. I thought it was fast-paced and fun, but a bit predictable.
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X8: Drums on Fire Mountain is one of the best "alternative setting" adventures out there. It takes place in a tropical island setting, isolated and detached from the rest of the game world, which makes it easy to insert anywhere in nearly any campaign. The action starts when the party is commissioned by the Sea Merchant's Guild to investigate the disruption of their trade route, and eventually sets them against a cult of green-skinned humanoids who worship a pig-like god.
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Man. Now I'm going to have to dig these old glossy books out of storage tonight and re-read them. All.