FFI: The Marsh Cave, hell the whole Elfland section if I could be honest. It's one of the least important sections of the game and the Marsh Cave is one of several That One Level in a game filled with this trope... The Ice Cave deserves mention as well, largely for being filled with some of the most overpowered mooks in the game.

FFII: Oh man, there are too many to count. Maybe Deist Cavern, the Tropical Island and Mysidia Tower because they are terribly long, are filled with "Encounter Rooms" hold little value in loot, and the first two could be easily written out of the game, and thus feel more like padding than anything else.

FFIII: Goldor's Mansion, because it has those damn horse enemies that confuse your party, and the Cave of Darkness because you have the splitting enemies, high encounter rates, said enemies tend to always ambush you and can do ridiculous damage to non-over-leveled Knights. Did I mention its seven floors deep and involves hidden winding pathways?

FFIV: Part of me wants to say the Sealed Cave but I've learned to deal with it. The Sylph Cave is another story with damage floors, hidden paths, two exits, and some of the most obnoxious enemy encounters in the whole game.

FFV: I never cared for the Pyramid of Moore, very few dungeons in this game really left an impression on me to be honest.

FFVI: Fanatic's Tower, I believe, is the sole reason why the Moogle Charm exists. Other than that, I would say the Cave of the Sealed Door is easily a dungeon I dread going through due to making sure I don't miss anything like the Ultima Weapon but mostly because it has some of the most annoying enemy encounters in the game.

FFVII: Fort Condor, I really hated the RTS game cause it's way too slow, thank god it's mostly optional. The Great Glacier is also super annoying especially since I have to do that long winded quest to get my fave summon Alexander. I also dislike Wutai.

FFVIII: The Deling Sewers and Esthar City during the Pandora mission, also the Ragnarok.

FFIX: Like FFV, very few dungeons left an impression on me.

FFX: Spira... oh that doesn't count? Well then Mi-Hien Highroad, Mushroom Rock, Thunder Plains, The Calm Lands, Cavern of the Stolen Fayeth, the Bevelle Cloister of Trials, Mt. Gargazet, and Inside Sin.

FFXII: The Henne Mines, largely the Zodiark Section, Tchita Uplands and Cerobi Steeps, partially because they are boring to look at and felt interchangeable, I hate the treasure farming in these regions, and I largely felt they existed for padding.

FFXIII: It would be easier to list which places I did like.