Frog God Games presents a mega dungeon that I will always recommend. With more content than you can likely get to in the next decade of gaming Rappan Athuk is a huge tome of adventure a massive sandbox that I recommend. Originally released for 3rd edition it has seen many editions over the last 20 years and is available currently for fifth edition, Pathfinder and Swords & Wizardry at the Frog God Games website.