What are you going to shoot at, and how far? Plinking and critters are two completely different things at their extremes. One is a 600m precision rifle, scoped with bipod, the other a hunting rifle in an alternate caliber with red dot. Neither fit in well as a tacticool quad railed blinged out CQB M4gery.
So, what the primary target will be, and how far, will determine the caliber, and that determines the barrel, which sets up what gas. An A3 upper will do from there, choose the reticle and magnification needed. The furniture goes back to what and how far? If precision or hunting, a fixed stock does fine, if you plan on wearing armor, you need adjustable, for when you aren't. Then the handguard is set, varmint or precision, a free float tube, hunting and tactical, regular handguards, and a quad rail only if it's issue. Nobody else needs it, and so says the contract supplier, KAC.
About dead last is trigger, unless the barrel, free float, and ammo are high precision, a trigger is just a feel good option. Literally. A high dollar trigger delivers almost zero improvement in accuracy, simply because the barrel has to shoot fractions of a minute to even see the difference. Otherwise, it gets lost in the standard 2MOA "good enough for combat" dispersion.
The target and range is exactly why there are so many different variations on the M16 design, from 24" precision shooter, 20" combat rifle, 16" hunting carbine, to 14.5" M4gery. Narrow down what the target is at the working range, and then drill down the choices by the rule of 85% - go for the better parts that fit the job 85% of the time, and seem to offer 85% of what the best, most expensive choice could be. You get a working rifle that fits almost all of what you plan to do, with great quality, and it won't leave you broke.
Go about it the other way, you get a bunch of uncooperative parts meant for different things that can't work together and do nothing well. Pics of those brag list guns are posted daily.