Despite the fact how much I despise DS there is no room for it in the top 3 worst instances.
1. Naxxramas 2.0
The place I hate most is naxxramas. I understand that people love it, since only a handful of guilds cleared it back in vanilla, but I just hate that place. On difficulity spectrum it was god damn awful: either you steamrolled through it half naked with the green items from quests or you tried Immortal/Undying only to rip off your own heads when priests failed to MC the adds on Razovius/ fucked up the Heigan disco/died from Frost Blast/Void Fissure at the very finish. The design and the music were making me sick as well (altough as I mentioned it is only a persnal taste)
2. ToC
Nuff said, despite having no trash the bosses were way too easy aside from Anub, which was a nice fight but imo not that worthy for being an endboss. ( Oh and f*ck you Faction Champions for focusing me out ALL THE DAMN TIME. There was never a single attempt when I actually survived).
3. Throne of the Four Winds - Al'akir Edition
While I agree the fact the Council was awesome ( loved dodging Rohas' bullsh*t) Al'akir was horrible. And when I mean horrible I mean it like the most vile, malicious luck based garbage that the world has ever seen. Alas, it is the Heroic version, when P1 was based on PURELY (notk idding) luck, and if you could get through it, you pretty much got the fight and killed it. But the first 20% was the hell itself