Skirt, though heels never have done anything for me, but that's just me. I know many guys like women in heels. Jeans and tshirts are nice too. A suit or dress are suitable for formal occasions or going out to a fancy place.
it all depends on how it's fitted. blue jeans and a t-shirt can be sexy but it can also look really sloppy. a dress or skirt can also be sexy but there are times when they just look bad.
It really all depends on what the guy is fantasizing about in that moment. If he just watched a movie with a sexy girl in blue jeans and a t-shirt he may be more likely to choose the girl wearing that.
I'm a simple boots-jeans-tshirt kind of guy and so I find it really sexy when a girl wears a tanktop or shirt with a pair of genes and sneakers. That said skirts on women are really sexy so a skirt with a top is just as attractive. It all depends on the girl and occasion.
Though call as I find both sexy, if I had to choose though, I'd say blue jeans, t-shirt, and sneakers. It shows me she can just be "chill" and not always "on the hunt" or feel the need to always dress up.
I'm not a dude, but it's obvious it depends on where the girl is going, considering the attire you described are for 2 completely different occasions. Also, how good does the girl make the clothing look? Attitude and body shape matters to determine what is sexy.
This for example link Would probably look sexy on a different shaped body.