AR-15s

Holographic is fine too. I would not buy one of those though, those are el cheapo and can break easily.

I would recommend Eotech or Holosun, Leupold etc.. They are pricey though.

Agreed. I had bought a Sig Sauer Romeo red dot, which is made by Holosun, it just felt too small looking through it. I replaced it with a Trijicon MRO, which then got replaced with an EOTech, but like you said, they are not cheap options, but I dont worry about them failing. I do have a set of Troy Industries flip up sights for backup, just in case.