I believe the question is too open-ended.
2 words need to be defined before that question can be answered.
In your mind, tell us what the word "best" means to you.
And in your mind what does the word "custom" mean to you?

As a gunsmith (not taking on any new work now and will not be for at least 4 more years, so this is NOT a slues pitch) "custom" means a stock that perfectly fits the owner. Drop at heel, drop at toe, cast-off, pitch and length of pull as well as the shape of the grip and forearm that suits the customers exactly as well as his choice of material or wood. As far as actions go it is not limited only to only newly made, but for reliability in my opinion, no one ever has made a superior action to a good Mauser 98, Controlled feed M70 or Enfield 1914/1917. And then there are the left handed customers. For them the "best" may be a CNC made mirror image M98 Sporting action, but in all honesty a LH Ruger M77 left hand is probably as good as such a CNC action because both should be set up to perfectly feed and work once they are barreled and chambered, so both should have custom work done before they are ever stocked.
On the other hand, "best' may mean accuracy and/or power is more important to the customers and having a rifle that seems as if it's part of the body may not be an issue to them.
So in that case a Desert Tec rifle may be "better" then anything I could make. They will "customize' to you wishes to some extent too, but I have trouble calling any Desert Tec rifle a custom rifle. Just having factory options doesn't constitute "custom" to my mind. Others disagree. Their opinion is just as valid as mine.

If someone hunts hard and carries a rifle for long distances but their real-world shots are at 400 yards and closer, this rifle is likely to be a better design.

[Linked Image from live.staticflickr.com]OLYMPUS DIGITAL CAMERA by .com/photos/156296479N08/]Steve Zihn, on [bleep]



So best and custom need to be defined by the OP. Mission statement of the intended use is key.

Every one of the rifles in these pictures is "best" to their owners of them because they are exactly what they wanted for the purpose they intend to use them for.
35 Whelen on a Springfield 1903
[Linked Image from live.staticflickr.com]20110928_2 by .com/photos/156296479N08/]Steve Zihn, on [bleep]
Custom 404 Jeffery made on a GEW Mauser with drop magazine and hand made stock that fits the owner perfectly.
[Linked Image from live.staticflickr.com]OLYMPUS DIGITAL CAMERA by .com/photos/156296479N08/]Steve Zihn, on [bleep]
62 caliber German Flintlock Jaeger rifle used for killing whitetails and elk in Montana Order was for a family heirloom to pass down to the grandson
[Linked Image from live.staticflickr.com]PB Jaeger by .com/photos/156296479N08/]Steve Zihn, on [bleep]
[Linked Image from live.staticflickr.com]PB Jarger 4 by .com/photos/156296479N08/]Steve Zihn, on [bleep]
and for those that want the best shooting machine they can buy but care nothing about the art of gun making, option for the science of gun making instead here is likely to be one of the very best options


https://deserttech.com/rifle-builder.php