Troy Industries manufactures the best iron sights on the market.
Favored by Special Forces units, these are the sights you need when failure is not an option.
There are a few things to consider when choosing your sights:
Fixed or folding (flip-up): Fixed sights are fine if you don't plan to use optics. Flip up sights flip out of the way for use with optics.
Standard, gas block, or micro height: If you're mounting the front and rear sights on rails that are the same height, then you would use a standard (same-plane) front sight. If you are mounting the sight on a gas block that sits lower than your rear sight, then use the sights designated as "gas block" sights. Micro sights are "same plane" but much smaller.
Tritium: Tritium illuminates your sights, without the use of batteries. Great for night time shooting.
Aperture style & Sight Style: Troy offers the standard round "peep hole" on most rear sights, but they also offer the "DOA" sight that has a diamond shaped aperture. For front sights, you can choose M4 style or HK style sights, depending on preference.