For me it's too broad a question; it depends what kind of dress / jeans / other we're talking about, and what kind of body shape.
About all I can say is that I am someone who is turned on by feminine legs and/or a round butt, so anything that hides those features is inherently less sexy. Baggy jeans would be up there as one of the least sexy things a woman could wear. (And of course I'm just saying what I find more sexy as per the OP, not saying it should be women's goal to look maximum sexy at all times).
