Just beat Pitioss Ruins, one of the hardest dungeons in the game, purely because it's a 100% platforming dungeon and it's tough as hell. It's not particularly crazy, just rather convoluted at times, but because of the characters normal momentum, it can be pretty hard to get around. I think I found like over 2 dozen accessories in it, along with the reward being an accessory that allows Noctis to automatically evade all attacks other than the AOE ones that'll hit you and if you are mid attack. You also get a bunch of items, so it was a nice restock. I can't say it was worth such a ridiculous time investment, but this is some serious endgame content lol
*EDIT*
Literally right after I wrote this, I went to leave the area, which requires getting into the flying car you get for beating the game and if you crash the car landing, it's a game over. I went to land, was going to fast, the car crashed, I realized that the past 3 hours, I lost completely. I was so mad at myself for not saving after that lol This game has pretty reasonable checkpoints, it's not the games fault, but man, I felt so stupid. I'm redoing that now, though thankfully, now that I know where everything is, the paths to take, I should be able to just beat it in like half an hour.
*EDIT*
Nope, still took like 2 hours. I'm never doing this place ever again lol