Fav: Yoko/Miku
Least fav: Any with some irritating catch phrase that they use at the end of sentences. Bonus points if they span it out, making them sound like some form of alarm clock. Only exception is desu.
Well, I got lots of gemales from all the animes I've watched so far... the girls from Clannad, Haruhi, Lucky Star and Horo to name a just a few. But, my absolute favourite is definately Nagisa from Clannad. Don't have a least favourite one, since I like 'em all.