I was obsessed with dark brown liquid liner in high school. Years later, I've converted mostly to pencils, since I like subtle waterlining, but Benefit's new Magic Ink Liner ($20) is tempting me to switch teams again.
It's a highly pigmented black that doesn't gray out when it dries, I can waterline with it (although it does sting for a second), and it didn't flake at all, let alone smudge. The brush is also ultra tiny, which is good if you want more control or aren't used to liquid. So if you're tired of black liner that isn't really black, or you're done with the inevitable under eye smudges from pencils, Magic Ink comes highly recommended.