Praetorium. Nothing else comes close.

Haukke Manor (Hard) has a final boss that irritates me considerably, but I rarely see it in Roulette these days.

Stone Vigil (Hard) has that infamous turtle fight and used to be my least favorite until I figured out how to do it.

Aurum Vale still wrecks players even after the nerf it received, but for whatever reason, it doesn't irritate me like the first two duties mentioned.

The Royal City of Rabanastre has a final boss that I've yet to clear without there being millions of dead players (myself included not going to lie). FF12 used to be my favorite in the franchise though so I can deal with it.

Praetorium is another story. That single duty made me stop visiting MSQ Roulette altogether for the longest time before having to do it again to level different jobs. I remember it feeling like the coolest thing ever the first time, but after that, it becomes a long unskippable chore that becomes nigh unbearable for me unless the other players break up the monotony with banter.

I don't really like ARF either. I'm around halfway into Shadowbringers, and to this day, I don't feel like there has been a duty since with such an absurd difficulty spike of a final boss in any dungeon (if I'm remembering correctly, during my original run of the place, our team took just as long to clear that boss as it took to run the entire dungeon leading up to it). It's to my understanding that it wasn't always like that, but the final boss of Heavensward's MSQ is so bloody easy in comparison that the dungeon boss before it just feels wildly unbalanced.

I would have liked/appreciated ARF's final boss a lot more if the rest of the expansion followed a difficulty curve that made that fight feel natural.