Its like Barbie for men (and a lot of women)
One awesome thing about the AR platform is the ability to customize it however you want. Want a flat top? The worlds your oyster. Want a different grip? Five minute swap. Stock not sticking? Pull Tab A, remove, replace, have fun.
When it comes down to a lot of the remainder of the rifle, its on personal preference. Me? I stick with 16 inch barrels myself. There is really nothing to be gained on a 20 inch barrel as opposed to a 16 inch barrel unless you are going for long range accuracy. You really only gain about 100 or so feet per second more going up from a 16 to a 20. An 18 is a good compromise, but again, theres not going to be any noticeable difference in the two unless you are, again, going for long range accuracy. And that requires your stainless barrel and a whole hoop of other items.
But back on point, there are literally dozens of types of grips, dozens of types of stocks, dozens of different kinds of handguards, sights, triggers, trigger guards, optics, muzzle devices and more. What works best? Eye of the beholder.
Sights. Whether or not you go with flip up sights depends on the optic used. With a standard rifle scope, flip ups are almost impossible to use. With a scout type scope, they become much easier. A red dot sight was designed with flip ups in mind, so they are a safe bet. I wont go into particular brands, but generally the big names in the industry tend to put out some decent sets. However, Id avoid any gimmicky types with crazy glow in the dark crosshairs and stick with the standard peep sights.
Optics. Most can get by with a standard red dot type optic and never look back. Magnified scopes are nice for precision work, though a good red dot or holographic scope will be perfectly fine out to 300 yards. Again, depends on the intended use of said rifle.
Stocks. Again, whats comfortable to you. The very first thing I toss in the parts bin is the standard AR stock. Unless of course youre dealing with a fixed stock variant. But collapsible stocks are pretty much standard and have endless possibilities with mil-spec diameter replacements. I tend to swear by B5 Systems and Magpul. But not to say there arent other brands out there that work. However, avoid Tapco like the plague. Or any cheap knock off that gets shipped from a warehouse in China. Or gimmicky types that really dont make much sense.
Grips. Always whats comfortable to you. Some use the A2 grip and never have a problem. Others swear by their chosen grip as its the greatest thing since sliced bread. I have my personal preference as its what fits my hands the best. So be sure to try different ones out before buying if you can. And stick to known name brands.
Muzzle devices. Most folks can use an A2 type device and never have any issues whatsoever. Others might want a brake to help with recoil. Others see the cool factor in some medieval looking nonsense and strap that hunk of steel on. If I was to suggest anything, this is one of the last items Id replace. Stick with an A2 in the beginning and replace as you desire later.
Triggers. I covered this already, but its worth mentioning again. One thing about the fire control group (as its formally known) is to be wary of the unknown manufacturers that have a cant miss deal with triggers. I stick to known brands like ALG Defense, Geissele, Wilson Combat and Timney. If in doubt, do some research. Like in depth research on something you think about buying. Dont go with the first review that pops up.
And magazines
there are always new magazine manufacturers out there coming up with the next best thing! Theres three different types of magazines out on the market: aluminum, steel and polymer. Aluminum mags I tend to stick with Brownells, D&H, Okay Industries and NHMTG. Steel Ill stay with E-Lander, H&K (pricey though), ASC or PRI. Polymer stay with Magpul and Lancer. If it says Tapco, walk away. If it says ProMag, pour gasoline on it, strike a match and run. Be extremely wary of new brands of polymer mags that hit the market. Buy once, cry once.
Wait, wait, wait! What about that rail?!
I didnt forget that. First you need to determine whether or not you want to go with a front sight base (A frame) or a gas block. A gas block ends up allowing the rifle to be smooth all the way from the charging handle to the muzzle. A true flat top upper receiver. This allows magnified scopes to be mounted and used. Or standard red dot optics with a flip up front sight. Additionally, the rail system, or handguards can extend to nearly the muzzle as well. Depending on what you want, this can be helpful in mounting (or not) additional goodies on the front end of the rifle.
Now an A frame is practically bulletproof. I have yet to see one that broke (so long as its pinned to the barrel) from normal use unless its driven over with a tank. Some cheap imitations that dont pin can (and inevitably will) loosen over time and cause cycling problems. But when it comes to reliability, the A frame cannot be beat.
Now the rails themselves. The standard and what most manufacturers use is the 1913 standard attachment system. Its a ladder looking deal on the handguards. Or others have sections that can be placed at certain points and moved around as needed. There are really three types of mounting points currently in use: The standard 1913 system in which the rails are already there (or screwed in), the Keymod system (looks like a big circle with a little on top) and the Magpul M-Lok system (looks like a longer slot in the handguards). Again, personal preference in which you buy, but accessories can get expensive over time. Again, stick to known brands and get the reviews on which you are looking to buy.
Im sure I havent covered everything, but its a decent start. If you have any additional questions, feel free to ask as there are plenty of us on here that can give you advice.