The AR-15 is the most-optic'd rifle in America, and the "best" setup depends entirely on your distances and role. Three approaches cover almost everyone.
Red dot: speed and simplicity
A red dot is the fastest, lightest, most reliable and most affordable option, with battery life measured in years. It excels from arm's length to a few hundred yards and is the default for a home-defense or general-purpose carbine. The limit is target ID and precision at distance.
LPVO: one optic, whole envelope
An LPVO gives you true 1x speed and dials up to 6x-10x for reach and target ID — the most versatile single optic. It weighs and costs more than a dot. See the LPVO buying guide to choose magnification, and ACOG vs LPVO for the fixed-power alternative.
Red dot plus magnifier: flexible middle ground
Keep your fast dot and add a flip-to-side magnifier to reach mid-range on demand — covered in do you need a magnifier. It is heavier than a bare dot but keeps 1x speed as the default.
Don't forget the light
Any defensive carbine needs a weapon light to identify targets in low light. Whichever optic you choose, mount it at the right height — see the mounting guide — and prefer a red dot or holographic if you want the simplest, most robust setup. Compare red dot vs holographic to finish.
