Well, I don't have any assault rifles, but I do have some semi-automatic carbines and rifles.
I like an Aimpoint or EoTech.
I prefer the Comp M3 to the Comp M4 because I don't think the extra weight and cost is worth the slight increase in ruggedness and I want smaller, not larger batteries.
The Micro is a little too small for me, but lots of other people love them.
I like the EoTech XPS models for their wider field of view. My HD AR has an EoTech XPS on it while my SCAR 17 has a Comp M3 and Magnifier on it.
The SCAR is just a target rifle for me, as are most of my rifles, but the AR is my go-to gun for HD.