Least favourite: Restoring the Thieves Guild with loads of the misc quests. Save / Reload to get the right hold.
Most favourite: Storming the D.B Sanctuary when you find it is under attack and finding Astrid sprawled across the floor burned quite badly --- Joe Flacco and the Ravens offensive incapabilities destroyed this sig.
Most favorite? The variation of builds I can play.
Least favorite? Two things. The fact I have to do certain quests to get all the Words of Power, or have to be made into a certain thing in order to complete the questline. (Wouldn't be a problem if it wasn't bugged out. Though it does nothing in actuality, the mere mention of it from guards takes away some of the fun of playing a character that hates Werewolves.)
And the other thing: Having to literally use every skill in the game even though I play a character that would never use such. Case in point, my current pure mage who only uses Illusion, Conjuration, Destruction, Restoration, Alteration, and Enchanting. The perk trees can be filled out nearly entirely with 80 perks, or at least, how my build uses it. Problem is, I only use 1/3rd of the skills in the game, thus forcing me to level other things the character simply doesn't use.