Ice Cave in Final Fantasy, in the versions where you can't save everywhere (i.e. NES, WSC or PSX). The difficulty of this dungeon lies in the enemies having instant kill moves, and you don't have good protection against them yet. It may happen wherever in the dungeon that you get ambushed by 9 Socerers, who paralyze your entire party and use their physical attacks, which randomly cause instant death, to wipe out your party before you get a single turn. Alternatively, you may run into a group of Mages, who ambush your party and cast Firaga and Scourge (instant kill on everyone), once again, before you get a single turn. Other enemies include 9 undead monsters who paralyze your characters (again, tough if they ambush you), and several Frost Wolves who use ice storm attacks to cause damage to everyone (hurts a lot if there's many of them and they ambush you).

As for the dungeon layout, you first navigate through the cave to get all the items, then you pick up the Levistone, and finally you'll need to travel through half of the dungeon again to get out of there. Usually I just get all the treasures first, exit and save, and then I speedrun to the Levistone and exit, hoping I won't run into an unlucky ambush.

IMHO, no other compulsory dungeon in the FF series comes even close in terms of difficulty, although FFIII and FFIV have some really harsh dungeons too, especially when you have to face tough enemies for a long time without a chance to save.