If the purpose of published modules is as much to provide a model for inexperienced referees as to provide a ready-made adventuring locale, then The Isle of Dread is certainly one of the most influential modules I've ever read. First published in 1981 as part of the Cook/Marsh Expert Set, module X1 is part of what I sometimes call the "Moldvay Pulp Fantasy Trilogy." Not a trilogy in the literal sense, these three modules -- B4, X1, and X2 -- all draw clear inspiration from a number of pulp fantasy authors and stories, X1 being an homage to Burroughs, Haggard, Doyle, and Merritt (not to mention King Kong). Being a huge fan of the "Lost World" genre of pulp fantasy even then, I absolutely adored The Isle of Dread and can't even begin to count how often I used it in my old campaign.
My love for X1 isn't based solely on its pulp fantasy roots. Indeed, one of the primary reasons I love it is because it's an archetypal location-based module, a format I prefer above all others. There's an exceedingly thin plot to The Isle of Dread: the PCs find a treasure map and, if they elect to pursue its instructions, find themselves on a far-off tropical island filled with all the Lost World staples -- primitive natives, monstrous wildlife (including dinosaurs!), inhospitable terrain, ancient evils, and wealth galore. It's a terrific set-up, both as a backdrop for adventure and as a tutorial on how to construct wilderness adventures. I remember absolutely adoring the players' map to the Isle, because it only showed the hexes immediately around the coast, with the interior being completely empty, awaiting the characters' own explorations. It reminded me of 19th century maps of Africa, which I suspect was no accident.
Like most modules of its time, The Isle of Dread is readily customizable. Even with all the encounters included in its pages, there are many, many areas that receive no attention whatsoever, allowing the referee plenty of room to incorporate his own ideas. I myself did this on numerous occasions, the most ambitious being my placement of the Forbidden City in the center of the Isle rather than the Kopru temple described in the module. To my young imagination, the Isle of Dread needed snake-men, so it only seemed natural to mash together modules X1 and I1. Given that David Cook was involved in both of them, it seemed to make even more sense. This is the origin of my still-percolating The Forbidden Isle project.
I wouldn't go so far as to call The Isle of Dread "perfect," because it's not, but it's nonetheless a very good module. Its greatest virtue is in being a mini-sandbox where some basic features have already been constructed as aids to the novice referee. Those features are sturdy enough to stand on their own merits but none is so impervious to modification as to prevent a referee from altering (or eliminating) them to his own satisfaction. That will always be the mark of a good module in my opinion and The Isle of Dread has it in spades. I'd love to see modules like this again.