The Bolt Carrier Group in your typical AR15 is where is all complex moving parts operate. The BCG holds the bolt (7) and it's associated parts, the firing pin (2), the cam pin(3), the gas key (6) and the firing pin retaining pin(1).
Other than the barrel, the BCG should be one your best options to upgrade your AR15.
Most of the BCG options out there center on materials and finish. BCG's are available in plain mag phoshate. That is a BCG with just a basic finish (mag. phosphate) that will protect the BCG from corrosion. It is by no means rust resistant and you do have to treat a mag phosphate finish with a product that lubes and protects it. The mag phosphate finish basically holds the protection (lubrication) on the parts longer.
So we will looking at, material, finish or treatment and testing.
BCG's are usually made from 9310, 158 Carpenter & 8620. These might be important considerations but frankly the material of the actual bolt is more important. The finish of the BCG is usually the factor most people lean on.
Finishes can be mag phosphate, chrome, nickle boron or nitrite. Of these the nickle boron seems to be a popular choice as a finish because it can reduce wear and makes the BCG slipperier. Any complaints people seem to have with NB BCG's is that the coating highlights any imperfections in the BCG base metal finishing. They are easier to clean compared to mag phosphate finished BCG's.
Chrome plating exhibits the same properties as nickle boron to a degree but can be less durable.
Nitrite finishes actually harden the surface of the metal without making the entire piece brittle. This same finish is done on (in) barrels.
Some BCG's have chrome plated surfaces in particular the interior of the gas key and the end of the BCG where the bolt rides.
For most folks and weekend shooters, a common mag phosphate BCG will be all they need. For those competition shooters and for military and LE, the choice is going to harder. At this point the details matter. What is the material, the treatment, the finish and has the part been MPI and/or HP tested? That's Magnetic Particle Inspected and High Pressure testing. Most high end shooters demand that their part be both MPI and HP tested. MPI looks for defects in the material while HP testing is actually running the part at a high pressure to make sure it doesn't fail. Can HP testing pre-stress a part? Maybe but at least you know it will pass the test once.
Were' almost to he point of TMI so use Google if you want more opinions. Remember, these are only opinions.