No budget options for barrels/hopup:
A. If you have time/patience/no rush because of corona
I personally would and am waiting a month or two until airsoft philosopher releases their new chamber: https://www.facebook.com/2080926842126379/posts/2608400799378978/?sfnsn=mo
They are a really cool company taking a no-compromises approach to building their product. What this means is, while they are very expensive, they are making the highest quality products to the highest tolerances you will find in airsoft. They have a really cool new innovative hop-up chamber with a perfect built in tdc settup that has not been seen before in the sniper community. They're making barrels, buckings, and hop-up chambers that are precisely designed for each other, and when we get some reviews/tests on their product after release, there is a chance that that will hands-down be the best build possible.
B. If you want to have the theoretically best possible build with current parts out now, that will take about 2 weeks, and save you a little money
If you don't want to wait, I would look into getting a lambda (one or five, doesn't really matter,) unbridging it, and getting a professional rhop install. That plus a flathopped autobot, assuming everything is done and installed perfectly, plus a shimmed action army chamber w/ a panthera nub, should give you close to the best possible performance out that is currently achievable (view first option for potential rebuttal).
Obviously these two suggestions don't really account for any sort of budget, and are likely a little unnecessary, but hey, if you have no budget and want the theoretical best rifle you can build, there they are... standard ml buckings + aa chamber & barrel should give you results pretty close though I guess. Just either lacking in consistency or airseal. Imo, every inch off target counts.
Bulltrigger makes the best trigger right now. More fps for a lighter spring, and it's probably the smoothest on the market. For the cylinder, everyone agrees that laylax and Action army do great cylinders. I would just run with one of their cylinder+nozzle. Both are identical, it just comes down to which you can get cheaper. Next, one of the most important and most overlooked things in the rifle is the guide rings. If you aren't getting a bulltrigger one-piece receiver or VSR-U receiver, I would go on skirmshop and get their new orange guide rings, or bulltrigger's guide rings. Either works fine I believe. Finally, for a piston, SAP or WASP pistons are great. The SAP will require you to use laylax springs, the WASP PDI or RAPAX.