Obviously the vestige himself. Single handedly saves every country in Tamriel and still has to pay for armor repairs, and the other self serving tools can’t even learn his name. WTF people!?
It’s the main reason I sacrifice the prophet every time. Well that and this whole mess is all his fault anyway.
The Telvanni, invariably. Minor annoyances is nothing compared to actually seeing people in cages, suffering. For the first time in ESO I felt I wanted to say "Sod the quests!" and go on a killing spree in order to free the slaves. Just like I had done in TES III. Since I couldn't do that (free the slaves), of course, it was the first time in ESO I'd felt really restricted. It's why I never bother with the Telvanni areas of Morrowind any more. I only really went there for the Magister personality and that's about it. I avoid that place like the plague.
So, yes, the biggest tools are the Telvanni, hands down. If you haven't been to the Telvanni areas of Vvardenfell yet? Go see for yourself. It's... pretty awful. I'm frankly amazed they don't have a Twin Lamps questline with how nauseatingly grotesque that whole macabre scene is.
Twin Lamps DLC, pls ZOS.