BigBL87 asked for things that are the best bang for the buck. While things like carriers with magic coatings are nice, they don't improve the performance of the AR much, if at all.
If the AR is properly tuned, extractor O-rings are not needed.
Things that given me the Best Bang for the Buck on My AR
-Lubrication. Mobile 1 Synthetic Oil. You get a whole quart of good oil for less than what you'd pay for most gun lubricants. The AR isn't picky about what oil you use, just as long as you use something. (I like Frog Lube better. It stays on the carrier longer and I like it being non-toxic and makes the rifle easier to clean. It's expensive and not needed to keep the AR running in tip top condition, but I like it well enough to pay the extra money)
-Grease the trigger. Apply grease to the sears, pins and pin holes and the bottom curve of the hammer. It usually makes the pull smoother. Next best bang for your buck is the ACT or ALG trigger. They are well made standard AR triggers, smoother and more consistent than factory supplied triggers. Standard factory triggers are a crap shoot. Buying an ALG/ACT trigger means paying a couple of extra dollars to make sure you don't get a crappy one. They are still very affordable.
-Sling. A sling is very handy to have on any rifle you carry afield
-Weapon Light. If your AR is going to be pressed into house defense duty, a light to ID targets is a must
-Reliable ammo. The frustration of ammo that doesn't work well in your rifle isn't worth any money saved
-Pistol Grip. Get one that's comfortable in your hand. Of all the furniture upgrades I've made to my ARs, getting the right pistol grip gave most noticeable improvement
-Magpul mags. Magpul mags will always work right out of the box. There is no need to add ani-tilt followers or worry about mis-aligned spot welds or anything else. Just unwrap, load and shoot. There are excellent aluminum mags bit there are some that are full of grief and you gotta know which are which. N o such worries with Magpul mags, even with their 40 rounders
Plan on getting an Aimpoint. They are not cheap but considering what they give you, I gotta give it the Bang for Your Buck Seal of Approval. I like the red dot sight and I can't think of any RDS I'd rather have. Long battery life, very rugged, easy to use and fast. If there is any gadget for the AR that has spoiled me, it's the Aimpoint Micro
PS- Try an H and an H2 buffer and see if one or the other helps your AR shoot smoother