Can't go wrong with an ACOG TA01, but it is a fixed 4 power. I put a USOptics 1-4 on my AR, but very pricey. Aimpoint makes a magnifier for there scopes, but I don't know much about them.
Most 1x scopes are not a true 1x, and distort image some, but that is a deep conversation I don't want to get into. You can have anything you want, what do you want to spend?