I've always had great luck with Magpul MBUS for backup sights. Watch the cheap crap on Amazon, it's made for airsoft.
This. Avoid the cheap junk sights on Amazon; it's pretty obvious which ones I'm talking about.
I've had great results with Magpul and with Troy/Midwest sights. I really like the Troy front sight with the AK style (round) hood, when paired with a red dot sight in an absolute cowitness mount.
For red dots I like the Primary Arms MD-ADS and Holosun versions.