Right now it really depends on your definition of ultrathin, ultrabook?
the Retina MacBook Pro (RMBP) is the BEST ultrathin, no questions asked. There is simply no comparison, the thinness is ultrabook thin, the weight is low for a 15, and the power is the same as the asus G75, I would go for the base model and thats it.
There is also this annoyingly good thread:
http://forum.notebookreview.com/what...ompendium.html
if you want to go for a 15'' there is a the S15 from sony and the HP envy 6t, 6z, and the 15