Depends on what you want.
In my opinion the best published dungeon crawling campaign is Deserts of Desolation (I3-I5). Chronicles of the Dragonlance is arguably as good, but requires a very sophisticated DM capable of generating creative content that supports the game when the players inevitably at some point get off the rails and making the game their own. The railroading techniques provided by the modules are simply too primitive, but probably necessary given the constraints of publishing.
The single best dungeon focused module is Ravenloft (I6).
The best pure dungeon crawl as game is Judges Guild's Caverns of Thracia.
I don't think there has ever been a great published megadungeon, but either Poltus (taken as a whole) or Castle Whiterock would be my recommendation. You could probably do about as well riffing off the old random dungeon generator in the back of the 1e DMG (expanding and theming the random monster lists for more variety and new level themes). Runner up here would be 'Axe of the Dwarven Lords', but its crippled by an over focus on goblins and a belief that Tucker's Kobold techniques represent good DMing. If you do run it, run the goblins until the PC's get tired of your crap, then have the surviving goblins run away rather than grinding it out with PC's well above your level.
In terms of challenging PC grinders, I6 Ravenloft, S2 White Plume Mountain, C1 The Hidden Shrine of Tamoachan, Mud Sorcerer's Tomb, S1 Tomb of Horrors and it's sequel Return to the Tomb of Horrors all come to mind. In the hands of a ruthless DM, you ought be able to kill scores of PC's with such tools.