[Fast and easy answer]
The Dungeon of Graves
3E just wasn't for me. I'm not hating or opposing it, we just didn't get along. That's it. No hard feelings. As a player, I don't care that much, even liked it on low levels (well, character creation takes it's time, but I see the appeal). And of course it is nice for players to believe they have more control over the game. But it makes a DM's experience of the game a lot more, well, mechanic? Less control, more fiddly parts, rules over rulings, etc.. Preparation sucked or you were to buy products with the work already done.
One of the more fun aspects of 3E was the shitload of third party products. Most of them were crap, to be honest, but there were some pearls among them. I consider Rappan Athuk one of those pearls. It's deadly and mean. Players had to bring their A-game or they'd get eaten alive. It was famous for being the deadliest dungeon crawl for 3E (and one of the deadliest of them all) and really didn't disappoint.
Incidentally it's also one of the few legit published Megadungeons out there. Sure, the OSR produced a few more (and still is), but before that? Castle Greyhawk, (maybe) Tegel Manor and Ruins of Undermountain. That's it. It's one of the best things D&D 3E produced.
Someday I'll revisit that dungeon with a few willing
victims players. Maybe with one of the newer versions? We'll see...
Also worth exploring: grodog's Version of Greyhawk Castle