I would say my dark indigo bootcut jeans.
Even after wearing them practically daily for several months now, they still look smart and brand new. They are such a flattering cut and can be dressed up and down, and most of the time when I have a night out (which is rarely) I end up falling back on them as I know they work.
If I don't know what else to wear, I know I can put them on and still look put-together.
Go ahead you can laugh all you want, but I've got my philosophy - Ben Folds