Which is best, most reliable, robust Linux being released to ARM CPU with CI set well for future development?

I haven't used it on ARM, but I would suggest Debian. I've used for years on AMD64 architecture and found it as stable as can be.

Just my two cents.

The board that you want to use it on may have a dedicated distro that the maker supports.

What board?
If it is a Raspberry Pi then there are several available for download from raspberrypi.org, including their raspi.os which is debian based.

Also, a quick search shows Ubuntu, Fedora, and Debian all with builds for arm64 and armhf architecture.

I use Slackware HF on my Pi3. AARCH64 is still in development.

I've had some experience with mobiles, and looked into related projects for arm. Almost ALL alternate OS or dual boot and images and such things were ALWAYS Debian, at least in all the hardware/projects I've looked into. I've seen projects with other distroes as well, but I've never seen one without Debian.

But ofcourse, don't forget about other more targeted projects like mer/nemo and such things as well.

I already looked up what an ARM CPU is (having had no idea), so I just learned something new. For others who read this thread but (like me) don't know enough to contribute, I'll post where I looked it up: https://www.arm.com/why-arm/architecture/cpu .

Maybe also post your learnings in the thread discussing new things you've learned.Have you done any research yourself? (Note that one member researched what ARM because they didn't know about it.)

As others have asked: What board? What specific ARM architecture? Most importantly, how much flash and RAM?

My opinion actually is that if you want something specific you may be best creating the distro yourself. I've done a few, and memory at times is extremely important. For instance one board had 64M of flash and 128M of memory. Not like you can just slap a regular desktop distro on there and call it a day.

Sorry the question is too subjective if you are only asking in general.

ARM is a certain processor architecture that a large proportion of processors worldwide (possibly the majority, the site suggests) use. Technical description aside, the practical import of the information seems to be that ARM processors are present in many types of devices--however, the type it does not list is personal computers. The list indicates small devices and large computers.

The Raspberry Pi line of SBC are the most used for normal computing, with several distros usable on them, plus they are being used in/to power other products - Debian/Devuan & Slackware are the biggest distros so far.