AD&D module S4: The Lost Caverns of Tsojcanth from 1982 has always been one of my very favorites. It's loaded with peak-era Gary Gygax flavor, and it's also really fun to play through. Doing so was one of the uncontested highlights of my first D&D campaign in the 80s, and I've revisited it (and the earlier tournament version from 1977, The Lost Caverns of Tsojconth) several times since. Having recently learned, via the El Raja Key Archive, that the Greater Caverns dungeon level has an even earlier pedigree, originally conceived as level 10 of Rob Kuntz's Castle El Raja Key c. 1973, and later incorporated as Core Level 7 of the legendary Greyhawk Castle itself, only strengthens my conviction that this adventure has the strongest claim of anything published of being The Quintessential D&D Adventure - the single product that best demonstrates and exemplifies the scope, feel, and intent of D&D as imagined by Gary Gygax.
That said, a close reading reveals some issues, especially with the first, outdoor section of the adventure (i.e. the material that was added in 1982). The scale of the map is too small, the caverns are insufficiently "lost" and don't show any real evidence of having been Iggwilv's former home, and the notion of a race to uncover them is played up in the introduction but not followed up in the adventure itself. Here's how I chose to resolve each of those in turn:
1. Comparing the outdoor map in the module to the World of Greyhawk poster-map, it seems pretty clear that the former is supposed to represent the entire southern section of the Yatil Mountains, from the hills of Perrenland in the northwest corner to the Velverdyva River gorge along the eastern edge - roughly columns C5 to I5. And yet, using the stated scale of 3.5 miles per hex, it isn't nearly big enough. One possible interpretation is to say that the map doesn't cover that entire area, but that's not aesthetically satisfying to me. I prefer instead to modify the scale to 7.5 miles per hex. This not only makes the maps match, it also makes the travel rates in the module (in hexes per day) more closely match those listed in the World of Greyhawk Glossography, so it's a win-win. While you're at it, go ahead and also change the scale on the outdoor map in WG4 from 3 hexes per mile to 1 mile per hex.
2. The adventure doesn't mention it, but the side-trail leading from the main trail to the caverns must be hidden in some way so that it's not obvious to everyone who passes that intersection, and con only be discovered if a party specifically states that they're searching the area for a hidden trail. Otherwise the supposedly "lost" caverns aren't actually lost at all - not least because, assuming the party is coming from the south, the trail leading to them is the very first intersection they'll pass. The outdoor portion of the adventure makes a LOT more sense if we assume the party initially passes by that trail and has all manner of red-herring outdoor adventures before eventually gathering enough clues and other info from the hermit, the gnomes, etc. to lead them back there, at which point they will probably feel a little dumb and annoyed for having walked right past it the first time.
A couple of sub-points to consider in this context:
a) The trail between the Caverns and Perrenland presumably saw heavy traffic while Iggwilv was in power there ("when Iggwilv ruled there was much coming and going to and from the caverns"); the trail south to Ket/Bissel and the trail east to the Encounter Area 1 and the Craggy Dells presumably saw much less traffic. Therefore at the first trail intersection someone with tracking skill (ranger, barbarian) may be able to discern that the trail north has seen considerably more traffic than the west and east trails combined, raising a question of where the additional traffic went. It strains credulity that this difference would actually be detectable, even to an expert tracker, ~80 years later, but it doesn't feel totally outside the realm of reasonable dramatic license to me, and makes for a nice demonstration of how superhumanly awesome such characters are at tracking.
b) Let's assume the Horn of Iggwilv is the tallest peak in this part of the Yatils. Based on a quick Google search it seems reasonable that such a peak might be visible from 50+ miles away. With the increased (7.5 miles/hex) map-scale that means it should be visible from the main trail starting at about the first "encounter dot." With the original (3.5 miles/hex) scale it should be visible anywhere south of the trail-intersection leading to Encounter Area 4. Some players (those attuned to the Chekhov's gun principle) will become immediately intrigued by this mountain once the DM mentions it; others will presumably make the connection after encountering the hermit. They may then attempt to strike out across the mountains towards it, or may decide to search for a hidden trail leading to it, possibly aided by aerial surveillance if they've managed to liberate the hippogriffs from the Craggy Dells...
3. It's sort of implicit in the published adventure that the Lost Caverns of Tsojcanth (i.e. the Lesser and Greater Caverns detailed in the adventure) weren't actually Iggwilv's headquarters per se, but rather were a location near her headquarters where she i) gained power by looting the more-ancient wizard Tsojcanth's trove of knowledge (presumably the spells that she inscribed into her Demonomicon), and ii) hid her treasure once she realized her reign was collapsing. So, accordingly, there must be another dungeon in the area that was Iggwilv's actual lair, but that isn't detailed in the module because it's already been thoroughly explored and looted. Per the intro to the module: "Cartloads of tapestries and rugs, statues and rare art objects have been recovered over the years as well as chests of precious metals, sacks of coin, and coffers filled with gems and jewelry. It was believed that all her treasure had been looted, and that no magic or wealth remained."
Furthermore, once the party finds the caverns, there's an empty/looted upper area that's dealt with in an extremely perfunctory manner in a passage of read-aloud text (and not even mapped): "The track into this area leads to a cavern with an entrance that seems like a fanged maw. The top is jagged and there are rising, cones of stone below. The cavern is 40' wide, 70' long, and over 20' high in the central area. It has obviously been used much in the past. The walls and ceiling are blackened by soot, and there are bits of broken furniture and discarded gear scattered around. At the back of the place is a smaller cave 20' wide, 30' long, and 15' high. At the northernmost end of the cave is a 10' wide passage slanting steeply down."
So, if one feels the adventure needs more of Iggwilv's Lair, the key to including it is just to expand that area to be not just one cave but one or more full levels, and instead of having the players automatically discover the back cave with the passage leading down to the Lesser Caverns as soon as they arrive, make them work for it. There shouldn't be much treasure on these levels, since they've already been looted, but it's reasonable that some monsters may have wandered back in (or never left). Perhaps if the players have had a rough time in the outdoor adventure and the DM thinks they need some more seasoning before taking on the Caverns proper, this area can be used as a training ground, to help them pick up some XP, gain some confidence, and improve their dungeoneering skills.
4. The last bit of cognitive dissonance raised by the adventure is how the canned introduction insists that the PCs must make haste and find the caverns as quickly as possible because agents from several other kingdoms are also searching and it's just a matter of time before someone finds them, but in the adventure itself there's no sign of any such groups - there are some patrols, but they're just guarding their own borders, not treasure-seeking. That, to me, feels like a lost opportunity, and that the adventure becomes much more exciting and memorable if there are several rival parties in the area and the PCs are in a race against them. I haven't fully statted these groups out yet, but I came up with the basic ideas for them, as follows:
Ket: This group is incompetent; a large (20+ member) party led by haughty, bigoted clerics. They'll spend the entire time wandering aimlessly in the wilderness, causing trouble for themselves and others. They'll have negative interactions with every other group of intelligent monsters or NPCs whom them encounter (except for the Kettite patrol, and maybe the hobgoblins), which may create issues if they get there before the PCs do (since the monsters/NPCs will have learned to be less friendly and trusting). Eventually they'll probably get wiped out by one of the tougher encounters, or starve to death.
Perrenland: This group has more knowledge (presumably there's more information about Iggwilv on the Perrenland side than there is in the south) and left to their own devices will eventually find the caverns, but they're too weak (max. 4th-5th level - say a magic-user (or perhaps savant) as the leader, with some fighters and a couple elves) to effectively explore them. If the dungeons are expanded to include "red herring" upper levels, that's where they'll stick to. Otherwise they might venture into the actual caverns, but if so will almost certainly be wiped out in short order.
Iuz: This is a smaller group of more powerful individuals - perhaps a high-ish level MU with a quasit familiar and some half-orc muscle (since in my campaign Falrinth is deleted from T1-4 [more on that later, most likely], this is a good opportunity to recycle him). These guys' strategy is to stalk the other groups and then swoop in after they've done the hard work. If they're able to follow the PCs to the caverns they're likely to wait at the entrance and set an ambush rather than venturing in themselves. If they come into contact, Drelnza will likely ally with this group (since they're in service to her half-brother), though not to the point of giving up her mother's treasures to them.
Veluna: This group (a couple of clerics, a cavalier or two, and some elf or half-elf scouts) arrives late. They could be a source of reinforcements for the PCs, if needed, but are also going to apply heavy pressure for the PCs to hand over the big treasure for the cause of Good rather than keeping it for themselves. As followers of St. Cuthbert they will certainly fight against the Iuz group if the opportunity arises, perhaps allowing the PCs to slip past them both with the treasure and avoid a confrontation...
Admittedly, the idea of using rival parties of NPCs to put pressure on the players is one of the same additions I suggested for WG6, but the fault for being repetitive lies more with Gary Gygax than me. And besides, I already said that I think the idea of a Great Race between the forces of good and evil to recover as many artifacts as possible makes a pretty cool basis for an entire campaign, so it shouldn't come as any surprise that I'm especially drawn to the two published adventures that already incorporate the concept (even if it's just window-dressing and neither one fully exploits it as-published).