Restoring the rightful king of Cumbria to the throne was a very short quest, but a very satisfying one nonetheless.
Bring the Dark Elf Z'an Al'urin with you when you tell Loghaire about the fate of the Black Mountain Clan. He'll start to get uppity about how he'll get revenge, and she'll deliver a verbal smackdown so epic Loghaire can't say shit in return.
Freeing the half ogre slaves and wiping out the elves at T'sen Ang can be very satisfying.
One here too. The dark elves tricked the dwarf king into sacrificing his clan, by masquarading as elder elf queen dwarves never met. The moment is when you can trick them the same way by wearing the insignia of assassin splinter cult that they sent hunting you into not only giving you information about their activities, but also getting the reward for your own head.