1. buy the most expensive/premium part for each category, assemble them together, and have a gun that fires like crap because even good parts can perform like crap if they're not the right parts for that specific gun or if they've not been assembled correctly.
2. decide on a specific area you want to improve (range/accuracy, trigger response, rof etc) then select parts that are the right fit rather than necessarily the most premium option.
lets take an example:
the stock nozzle on an e&l ak is actually pretty damn good, maintains very good air seal and has a little o-ring to help it along.
the combat union hop unit for an ak is very good, arguably the best out there. certainly better than the stock e&l hop
neither of these 2 parts are bad parts, however try using a combat union hop with an e&l nozzle and your gun will shoot like shit, because the nozzle steps out a little and this is too large to fit the combat union chamber, making the nozzle jam open meaning you get fuck all air seal.
now fortunately for that particular case combat union know this is a problem and include their own nozzle with their hop unit, but the point still stands that 2 nominally good parts can fail spectacularly when they're not a good match for each other.
this is why so often when folk tinker with their guns it ends up worse.
now the other problem is that of picking a specific area. the performance demands for a gun that's used in cqb are going to be different to those of a gun used outdoors. likewise a gun built to optimise accuracy has to accept at a certain point you're limiting the rate of fire potential.
the reason i'm saying this is to point out that the question is too vague, there's no guaranteed recipe for the best gun and top of the range parts are not always the answer. where we can help is if you tell us what specific aspect of the current performance you're finding isn't good enough and we can help you focus on the areas that will improve that aspect.
