A plain white t-shirt... someone could say that it's too boring to mention but I think that there's nothing more variable and stylish than a white tee. You can wear it in many many ways and you would never look the same if you combine it with the appropriate accessories.
Just a few tips for you...
With a skirt
A white tee looks incredibly perfect with a leather mini-skirt, in such a combination you can choose from variety of accessories to make your look more unique. It could be an interesting purse or shoes, just enjoy the styling... Another great combination is together with a long loose skirt, it looks fabulous with a wide belt or an elegant hat.
With a jacket
Simple, elegant, trendy... what to say more? An absolutely classic look - slim jeans, a jacket and a white t-shirt and you will never be wrong. Instead of a classic jacket you can wear a denim shirt and if you fancy shorts, combine it with a white tee and a jacket.
With pants - a classic
Just some other variations of a tee and pants... Don't worry to combine various patterns, the white t-shirt allows you to be creative :)
Totally fantistic combination is also denim and a leather jacket... In Summer you can wear the denim shorts with a cardigan and don't forget the accessories...