Paint marks on my windage and elevation dials for my mechanical zero reference.
A flat top is easier if you want to scope an AR15, but if I had to put an optic on the handle...well, it depends on what the intended use of the rifle is. One optic may be better for a specific or intended use than another; for instance, you'll be better off with a red dot sight like an Aimpoint over a Leupold MK4 scope for a close quarters use rifle.