U.S. Rifle, 5.56mm, M-16A1 / AR-15
I joined the United States Marine Corps a week before I graduated high school.
Back in those days, the M-16 had triangular hand-guards, and iron sights.
Call it nostalgia, call it what you will, but that is still my favorite configuration for an AR-15.
I also prefer the pencil thin, original profile of the barrel over later, thicker units. We trained to hit man-sized targets out to 500 yards with that barrel, and with an maximum effective range of only 430 yards, how exactly does that thicker barrel really help much, when I was already able to shoot that well with the skinny one?
I think they really screwed up the intrinsic balance when they beefed up the barrel.
Though, as I understand it, part of the reason was to facilitate a more robust front end for bayonet use.
As my Marine Close-combat instructor so poignantly told me oh so many years ago, a bayonet may be useful for hand-to-hand fighting, or as a last-ditch weapon when out of ammo, but the real reason for mounting a bayonet is simply, "Because it FEELS GOOD!"