Alliance's ony key questline personally; it's an effing pain in the ass going into BRD 2 or 3 different times and escorting him out but the ending makes it all worthwhile; meeting up with Windsor at the front of SW, walking with him through the city as all the guards stop & kneel for him, eventually confronting Onyxia herself in SW keep followed by fighting off the elite dragon guards right then and there.

Storywise I'm fond of the MIssing Diplomat questline as well, finding out the diplomat in question is the king of stormwind and chasing the defias all over trying to find him, and they finally added onto the questline with the addition of mudsprocket & more theramore quests, giving a little more closure to it. Having Jaina pop up near the end of the pre-2.3 questline is nice too, even though they're too late to save me from the ass-kicking lol