Without going into a long dissertation, I would suggest one of two brands:
FN or Colt Defense. Both have a very extensive history in making very high quality guns for the military.
In Colt I would suggest either the 6920 or 6720. The only difference is the barrel profile. The 6920 has an M4 type profiled barrel. The 6720 has a "pencil" type profile and is quite light.
Yank the trigger out, and place it in a ziplock bag, as a spare. Drop in a Geissele trigger.
Top with optic of your choice, either a low powered variable or an RDO (red dot optic) Aimpoint is the standard by which all other RDOs are judged.
Add a white light.
Finally get a good quality sling. I prefer 2 point adjustable slings like the VTAC. A VCAS, or BFG 2 point sling will work too.
Stay away from single point slings unless you like getting racked in the balls during transition drills, and or like a rifle that flops all over the place when both hands are busy with another task.
In summary:
Colt or FN
Geissele trigger
Aimpoint
white light of choice
2 point sling.
DONE.
