Guns: Son of Frankencarbine!
Going by the whole "two is one, one is none" philosophy, here's a carbine I put together to back up my main housegun, the Daniel Defense AR:
BCM Complete Upper (16" lightweight barrel with midlength gas system, VTAC ALPHA handguard, BCM bolt carrier group) - Very happy with this upper so far. Completely reliable, accurate, and extremely svelte considering that it has a full 16" barrel.
Spike's Tactical Lower (Zombie receiver, enhanced lower parts kit) - Pretty spendy version of a standard lower parts kit. I like Spike's "Battle Trigger" okay (basically a crisper, lighter pull version of a milspec trigger), but the competition-style units (Geissele) are what everyone uses nowadays, and I'd probably opt for that if I were to do it all over again.
Magpul stuff (CTR stock, BUIS, MS3 sling/ASAP plate) - Pretty standard equipment in the AR world; if you don't have them on your gun, you've probably tried them on someone else's gun. I will note that the MS3 is a way better sling than the old MS2. It's more comfortable (wider webbing) and easier to use (new all-polymer attachment hardware).
Quark AA² Tactical LED flashlight in Viking Tactics mount - I've taken to putting this combo on every one of my long guns that can handle it. It works especially well on the BCM upper: the flashlight is mounted so far forward that almost none of the light is blocked by the barrel.