Men's button-down shirts are a wardrobe staple for me.
They feel polished and "grown-up" enough without being at-all girly or 'frou-frou' and work for pretty much any occasion. I wear them with jeans to the office (we casual over here), with fancier printed pants and heels to events, with denim cut-offs to the beach, with sweatpants and sneakers on the weekends, the list goes on.
I already own a lot of these but I'm constantly on the search for more just because I wear them so darn much. I have ones in silk, ones that are sheer, ones in crazy patterns but for everyday use I prefer oxfords because they're 100% cotton (aka no dry cleaning), comfortable, and look okay wrinkled (as they inevitably are). And as far as basic oxfords go, this one from Uniqlo is pretty much the best I've come across. I already own it in several colors, and plan to buy it in linen when it gets really hot out.
They're really well-made (last for months even after loads of use) and fit nice--close to the body but not snug and just the right amount of menswear inspired. Oh, and it's $20. Seriously you can't beat that.
Get it here.