Well, first you gotta decide what the intended use is. Tactical Carbine? Varmint Hunter? CMP match shooter? Plinker? Target gun? Once you have it narrowed down, you can then choose your components. There are several top quality companies out there. I prefer Rock River Arms, but also like Armalite, Stag, Bushmaster, DPMS, to name a few. Rock River uses Wilson barrels that are very accurate. I also like the Rock River Two Stage Trigger in most any rifle configuration. If you're building a 16" Carbine, look at the Mid Length handguard/gas tube option. There are a ton of other options/upgrades, but as I said you first have to decide what the main purpose of the rifle will be.
Donnie