Getting even with the gnomish Industrial Council for running those vile experiments to breed Half-Ogres by helping Donn Throgg become the president of Tarant sure was sweet.
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.