Ancient Red Dragon
Orcus
A toad named Trevor
Actually, I practically never play spellcasters, so familiars aren't something I'm "familiar" with, har-dee-har-har.  I usually am on the lookout for something a bit unusual for the most part, though.  Improved Familiar is a must-have feat for me if I ever do, just because I like the concept of having a unusual and different familiar.  The mowz is a pretty good one.  I kinda like the little fiends and such too; the mandragora from Bood of Fiends, or imps or quasits.  There's something iconic about having a little punk like that as a familiar if you're going for that inscrutable, corrupted, typical Sword & Sorcery type wizard/sorcerer.