One of the beautiful things about the AR15 family of rifles and carbines is the accessories. Most gun owners like accessories and I am no exception. I own a whole pile of AR15s and most of them have something aftermarket on them. I buy the good stuff. I don't buy anything because it is cheap or ALMOST as good as ________ . Obviously this means I often have to save up for something, or work overtime, or wait until I get a windfall, but I would rather have the best stuff made than waste my money on inferior stuff.
That being said, most of the stuff mentioned is stuff I have never bought. All 13 of my AR15s have the stock pistol grip. I tried the "gapper" and didn't think it was worthwhile. The only time I ever got a hot spot on my middle finger is when I have taken a formal class and was firing in excess of 2000 rounds a week. During my normal shooting/training it isn't an issue. I do use the accuwedge in all my guns. It does nothing to make the gun more accurate. It just gives it a more solid feel. AR15s get loose with use. This doesn't effect their performance at all, but it gives them a weird feel that I don't like; so, I use the accuwedge. All my short AR15s have a single point sling made the The Wilderness with a Daniel Defense mounting plate. The Danial Defense mounting plate is angled down so you don't catch your fingernail on it when you are running the bolt. A couple have Knight's Armament Corp. RAS IIs on them (rail adapter system that is also freefloating). This allows me to use my Surefire 900 series weaponslight on any one of them and switch between them in seconds. For optics I use ARMs mounts. I have one AR with an Aimpoint ML2 on it, another with a Trijicon ACOG TA31F on it, one has a fixed 10X scope on it; all use ARMs mounts. One has an AO night sight on the front and a "same plane" rear sight on it. In my opinion, all these accessories are the best available, and are all well proven items. They have proven their value to me.
My two 20" guns are basically stock. I do have a Turner sling on my 20" Bushmaster and that is it. My 20" Colt SP1 is bone stock.
Following the advice of a lot of people that know what they are talking about, I stay away from aftermarket triggers other than the one I have on my 24" scoped "varmint" AR. On the short ARs I feel a short stock is madatory: no longer than an A1 stock and preferably shorter. Something along the line of 8" - 10" LOP. I really like the Sully stock but it doesn't allow me to use The Wilderness single point sling, so I use a standard A1 stock. All my shorties with fixed stocks have the A2 stocks replaced with A1 stocks. I like the PRI "Gas Buster" charging handles. On my flat tops I use an ARMs #40 back up rear sight.
One accessory that no AR15 owner should be without is the LULA magazine loader.