1. Pacman is by far the best package manager out of any distro in my opinion and the AUR is awesome as well, especially compared to the absolute mess of PPAs you'll eventually end up with on ubuntu based distros.
2. by far my favorite thing about arch is the control over your system that comes from how little it does for you. Yes, the install is one of the hardest, second only to Gentoo in my opinion, but it gets really easy once you've done it once or twice and after the install, it is not hard to use at all, at least not in my opinion.
3. The arch wiki is a great resource, even for non-arch-users.
4. Last i checked, Ubuntu shipped with almost 1600 packages. Arch only ships with around 300 i believe. Everything else is there because you put it there. On my fully set-up, gaming and programming equipped main rig, i have only a total of 1117 packages installed as of now.
5. That also means that arch is a lot faster and less resource hungry. Ubuntu booting from an nvme drive in my laptop can't keep up with arch booting from a fairly old hard drive in my desktop, and both have otherwise similar specs, except for the gpu. Program startup times are also a lot quicker on arch.
6. Rolling release. Self-explanatory
Also, arch only breaks if you break it. Yeah, sometimes smaller things stay broken for some time, but i've never experienced any major or system breaking bugs that i didn't cause myself.
That Arch is my favorite doesn't mean i'd recommend it to everyone though. Manjaro is a lot more suitable for people new to linux and still has a lot of archs benefits