I think modern high-end actions are so close you are just choosing on preferred features. If, by base you mean the "most critical" part of the build, I would have to say it is the human element in the chamber work and barrel. I am not saying they do not have amazing tolerances and finishes, I just think the heart of a build is barrel and chamber. When actions are all within several ten thousandths everywhere it gets hard to discern which is best. Is a Bat VR $400 better than a Kelbly Atlas? I don't know and I doubt I am good enough to find out. I do know there are people out there that swear by one or the other and that's great! It would suck if there was only vanilla ice cream!I've been going through analysis paralysis on an action for another build. I have been very pleased with the Defiance AntiX I have, but would like to try something different. The Kelbly Nanook and the BAT Vesper are on the short list. There are a number of great actions out there to build around. Rightly or wrongly I believe the action is the base of any build.
There may be lesser custom actions out there, but I do not think any of the brands listed here will disappoint.