My favorite addition to my several Colt carbines, top 3 in that order:
1. Light - my favorite is any Surefire model light, it had not let me down in the last 10 years I had been using several of them.
2. 2 point sling - this is the equivalent of the holster on pistols, I prefer my LaRue Tactical padded version when on the move (hiking, hunting, running, climbing) and my Vickers (standard & padded) VCAS when mostly shooting.
3. Red dot sight - my favorite is my good old Aimpoint or any same brand models, I had been an Aimpoint early user for 26 years now without any problems so far.
My other additions to some of my carbines after the first three:
Railed handguard or Magpul MOE handguards - to mount my light, vertical grip and sling QD mounts
Grip - mostly I use the Magpul MIAD grip because of the different sizes, for more vetical angle I have a B5 grip, and I don't change the standard Colt A2 grip for my M4 carbine and A2 rifle clones.
Buttstock - I've got different kinds for different uses and to balance my carbines, Magpul CTR & STR, Vltor IMOD & EMOD, B5 Bravo SOPMOD, and factory standard stocks for clones.
Free floating handguard - I like my LaRue Tactical 9 inch rail system on my precision carbine with a better Rock Creek mil-spec chrome lined, midlength gas system, medium contour 16" carbine barrel.
Ambi controls - most of my non clone carbines have ambi, selector, magazine release catch, and charging handle.
In the near future I might get either ALG or Geissele triggers, but for now I'm satisfied with my well used triggers that are more smoother now compared when they were brand new.