The best outfit for a girl is confidence. Clothes is 1%. Confidence is the other 99%. Of course, it's not very eye catchy to see a girl in a clown suit, but confidence is very very very sexy and attractive.
When you say clothes I think things publicly acceptable. I would have to go with something that shows you are not shy about your body. Thats what's sexy.
I do have my own personal favorites too. Wife beaters Basketball shorts Fitted jeans & shirts
Sexy is truly a state of mind. When you are getting ready play music that makes you feel sexy, dance to it. naked. Don't look at photos of girls you think look sexy. KNOW you are a sexy woman and you will look sexy in sweats or a mini skirt.