Acog from trijicon: pay once, cry, enjoy - Trijicon ACOG 4x32 Scope w/ Red Illumination Doughnut Reticle BAC-M16 / AR15

Yes, you can go cheaper glass...less durable... less everything.
This is top of the line. It is what other makes and models see as the gold standard by which to measure their products.
Clarity is superb. Magnification is spot on. Intensity of red dot is sharp around edges of the doughnut reticle.
Buy this and you will not be disappointed. Ok, you will have a thinner wallet, but you're only buying this once, instead of an inferior scope that you will replace in a few years anyhow. Pay the dime, versus three nickles.