Vampires for me, and it's not even close. I do love spell casters, and I have played several of them since RoM came out. I did several witches in TS3 as well, but I have a life long fascination with vampires and werewolves. I love movies, books, everything that tells a story around them, so having them in the game I'm naturally drawn to them.
For whatever reason, I have never encountered a spell caster performing spells outside of the Realm. Not even in Glimmerbrook. I don't know, do you have to be actively playing as one for others to appear and perform magic outside of the Realm? Or am I just so wrapped up in whichever sim I'm playing that I just miss it happening all around them? Whenever the subject of spellcasters comes up it brings this question back up in my mind.