I can't say I'd like either extreme.
I shy girl may be shy for a reason, like she's very down on herself and may be needy or and need constant reassurance. I don't want to paint everyone with the same brush, thats just some of my experience.
At the same time I can't say I'm very attracted to overly-confident girls. I like someone to have confidence in themselves, but I don't like it to the point that they are very in-your-face about it, need to be the center of attention, think everyone must love them...
Like I said, its a balance, either extreme isn't very sexy.