I'm probably an outlier here but I like the "mosaic" ones the most (these
http://t2.gstatic.com/images?q=tbn:ANd9GcSPFIJekGiOkVrufE7nLB5jjrMv0uqt4Mn_lYmBrh7B-6bIVfjx).
The face ones are, as others have said, very boring and generic. And while I like the Rankine ones in concept, and that they use some sort of material from the series itself, they still don't really do it for me.
As dumb as it sounds, I think the "mosaic" covers actually made me take the books more seriously from the onset (not that I wouldn't have after reading the first couple pages). They look...classy? I don't know, that's not really it. They don't scream fantasy -- or even really hint at it -- which I kind of like for whatever random reason. I generally prefer simplistic designs that evoke a feeling, rather than overt imagery, when it comes to most book covers.
Terrible picture here, but just as an example of what I mean, I love the front-end design on A Dance with Dragons, way more than the dust-jacket itself (which to me is basically just as generic as the TSA face covers, but slightly less offensive with the lack of face).
(it irritates the living hell out of me that the design isn't centered vertically though)
