Dark brown. The colour is a lot softer for me because I have natural blonde lashes with pale skin and blue eyes. Black tends to look super harsh. Unfortunately, a brown mascara is really difficult to buy and if you found one, they're never really good.
I have black but I also wear a special black with gold flecks made for hazel eyes (really makes the eye color pop). I have a brown that I wear daily as it is natural looking and rotate the blacks to suit my mood.