YEs. My spouse is my best friend. He was before we started dating, while we were dating, when we moved in together, when we started a business together and after we married (which was Saturday BTW!).
I tell him everything. In fact I feel the way you can tell who your best friend is, is it is the person you would call first to tell something important, funny, sad, happy, exciting etc. He makes me laugh, we enjoy each other's company, like the same stuff and are very much alike.