For 'default state that you buy them in' I had to put Tiffany, she brings the most to the Sim for me, although it was a very tough choice, they're all excellent models and with the ability to swap and change features every model has something of worth - hence it's alway a good investment to get new ones as they come out!
Honourable mention goes to the user-made character Sinead, as she's definitely my favourite personally, just not on the list for voting!