Personally the one I hate the most is where they take random mobs make them a bit bigger/faster, put them together and then call it a day. It feels like there wasn't even any effort put into making the boss and I've never seen anyone say "Oh yeah, I loved fighting that boss!". Most often it becomes more about either abusing their stupid AI so you can get hits in while they mindlessly spam attacks or use some combination of armor spells to make you shrug off their damage and zerg them.
It doesn't feel fun fighting them at all, but more like I just want to get the stupid fight over with and move on to a real boss that actually has real mechanics besides from attempting to mindlessly spam attacks and shorten your windows for attacking.
Only thing that can get close to the frustration of that is when the real boss is your camera.