Envy Demon, Malvernis from DA2 Legacy and the Sky Horror from Mark of the Assassin.
For all the praise Origins gets, I never found any of the fights particularly hard even in Golems of Amgarrak. (Rogue Warden can curb stomp the Harvester in seconds)
Same deal with the Arishok although that's more to do with Rogue Hawke being hilariously OP.
Yyyyeeeppp. All dex rogue = evade everything, tear him into a million pieces. Don't need the other party members. It was obviously balanced around coming in fresh so I imagine bringing in just about any decked out Warden would make the fight easy, even on nightmare.
DA:O - Nothing. I recently did two playthroughs with zFCR very hard tweak on nightmare mode and still found it kind of easy. Although granted I was using the uber items from Improved Atmosphere, but I don't think those particularly unbalance the game that much.
In those two playthroughs, the hardest part was the dwarf noble origin. Especially the fight with Trian.
Funnily enough, with zFCR very hard, the shrieks and shadow wolves using overwhelm basically one shot anyone, ESPECIALLY in Awakening.
DA2 - Malvernis probably. Also Corypheus. Malvernis mostly because phase 1 is pretty hard. Clever positioning can make it so you can pick off archers a few at a time without taking damage though. Corypheus because your party members LOVE pathing the EXACT wrong way during the 2nd and 3rd flamethrower phases.
Another interesting fight is the Arishok melee. This battle really comes down to whether or not the saarebas decide to nuke you. That lightning explodey ball of death doom does WAY too much damage on nightmare.
DA:I - Haven't beaten the game yet but the Ferelden Frostback and Knight-Captain Denam have been the hardest for me so far. Both because you're at a low level with bad gear and limited abilities.