Here's a general run-down of megadungeon products, in no particular order, that I'm aware of, with my quick impressions thereof:
- Caverns of Thracia - the best - buy it: a ruined city in a jungle with levels below; easily the best megadungeon out there if you like old school dungeon delving with complex maps and hidden sublevels and such; only complaint is that it's a bit too small to be a true megadungeon unless you expand it (much like NG's Tomb of Abysthor, the perfect d20 equivalent to CoT [excepting the d20 conversion of CoT, of course!])
- Rappan Athuk: I'm not too keen on RA as as stand-alone megadungeon, it's a bit too small for my taste and it's tightly focused themes of undead and Orcus make it a bit harder, but I'm happy to mine it for encounters and ideas; the free map downloads on the NG site by Chris Boll are the best RA maps published, and it's a shame that they weren't used in the RA:Reloaded boxed set (though you can download them, of course, if you have the original RA books' passwords too)
- WGR1 Greyhawk Ruins: this and Undermountain and RA form the standards against which other megadungeons are judged; like RA, its levels are a bit small for my tastes, but Greyhawk Ruins presents a large and complete megadungeon, ready for delving
- The Ruins of Undermountain I (the good one), II (the sucky one), and three stand-alone modules, along with The Fireplace level from Dungeon 128 (and perhaps the Dungeon of the Crypt, both from the Vampires of Waterdeep trilogy): I like UM for it's sheer scale/size, though Skullport's not really my style; you can also cannibalize UM since it's not a complete mega-dungeon either (though, like Maure Castle, it does provide an outline of the whole)
- And of course, there's always the Castle Greyhawk/Dunfalcon/Zagyg/El Raja Key/Maure Castle smorgasbord:
1. 2 parts Castle Zagyg: the Upper works by TLG
2. 3 parts Maure Castle (between the levels published in WG5 (original 3 levels)/Dungeon 112 (3 + 1 new level), and the two sequel levels in Dungeons 124 and 139, and Kuntz's freebie level on the PPP web site, you have a lot of content to fill out as dungeon levels below the Castle Zagyg first level dungeon
3. 1 part Greyhawk Ruins (pick out the good stuff, and you can probably use some of the maps too)
4. spice to taste with WG7 Castle Greyhawk, Expedition to the Ruins of Greyhawk, along with demi-planes from EX1/EX2/WG6 as required
I don't own Castle Whiterock (kicking myself for not picking one up last year at DunDraCon on the cheap...), or WLD (never got into it after all of the negative press, and the concept seemed far too random to me too---1 encounter with each monster in the MM or somesuch??), Ptolus (too expensive, though I do think that the BaneWarrens is Monte's best published d20 adventure), so I can't really comment on them as megadungeon producs.