Scar-L or MP7.  Perfect circle with a line in the middle.  Or, open site with no circle.

I like the mp7's there clear and easy to see through.

My favourite one is the MP9 sights. Reminds me of the TMP sights from MW2. So clean and easy to use. MP7 is another of my preferred iron sights.

I like the m4's sights i could never put a scope on that gun the iron sight is pretty sweet just the way it is.

IMO, it's the weapon you use the most.  For me, that's the P90.  I love the iron sight of that gun and can get easy headshots with it.  Next for me would be MP7, prob. my second most used gun and then the PP90m1. 

I think the best is one of the above mentioned but the worst is the M60.  I cant stand anything on my guns but iron sites and even after getting the M60 gold 4 or 5 times I cannot stand the irons.

They are so obstructive and block half of your view.  I have to use marksman with it to stay on target..  Luckily it has great hip fire.

IRL I have killed more deer with my  old 30-30 with iron sites then all of my other guns put together.  That doesnt count coyotes, antlelope, crows etc.  It was my great grandpas and my first high powered rifle.  Sure wish I could put irons on one of the sniper rifles in the game.

I like the UMP, hate the open MP7 sights; but with SMGs I hipfire mostly.  I feel like I move too slow with ARs or LMGs.

SCAR-L has amazing iron sights.  It's the only gun I use without a Red Dot.  The rest I find are horrendous.  Well maybe the ACR.  That has decent iron sights.