Most story boss fights since they're insanely long and non-engaging. Extra points to the Zenos fights you're suppose to lose, but also can't lose too early.
The entirety of Void Ark. Completely unengaging snore fests which is impressive considering probably less than 1% of players were even able to get gear out of Gordias. I'm convinced that this place would be a cake walk even if you did it with fresh 60 leveling gear.
Zurvan/A11, although this is more of a meta reason, but I blame these fights for the disgustingly disproportionate "dps>everything" attitude that still plagues the community to this day.