Helllo, when I had installed Fedora, I had too many problems with missing rpm packages, in EOS I need sometimes use AUR. Which package system the best, as for you, and contain the most packages?
Pacman and yay
Debian APT is easiest to learn.
I often see when for installing some software articles said adding one repo, then next repo…
So all apps in debian not available in the main repo(in EOS I never add new repo to list)
I’ve never experienced that in MX-Linux. I had three repositories (MX, Antix, and Debian stable), and most of the software came from MX or Antix. So I can’t really judge plain Debian, but with the MX distro, I didn’t have to switch between repositories very often.
The package system that best suites YOUR needs is the one that does the job you think it should.
Ok, which distro include the biggest repo: Arch, Debian, Suse, Fedora?
I would claim that it’s Debian, but could be wrong.
If we talk about official ones that is.
I assume so, too. Although I suspect that Arch, with the AUR, isn’t far behind.
According to this site:
The top one for raw packages is NixOS. Arch + AUR is not far behind.
If ignore AUR and NixOS?
zypper have similar count of packages as pacman?
Maybe but, pacman and yay provide more and varying packages/apps than APT does. 
Which is why I install distros that use pacman and yay. I dont have to go hunting for apps that I use and want. APT does not offer some of the apps that I like and rely on.
The question isn’t very fruitful. All distros have the packages to get the system going. Most distros provide the popular packages. After that you will always hit that one package that exist in one distro repo but not the other.
After that you run into situations that downstream doesn’t properly package upstream (see the OBS vs Fedora fight). Or a distro provides a ton of packages, but half of them become severely outdated (hello Debian). Or some knowledge/skill is required adding or reviewing a 3rd party source (shout-out to the AUR). Or some break and then sit unmaintained for months because the community is to small to maintain 100k packages (
NixOS). Or a bazillion other issues.
Imho it’s part of the distro-hopping experience
. Eventually you’ll find out what serves you best overall.
Snaps! 
pacman has a big advantage: PKGBUILD files are easily created for any software you find. Be it deb packages, tar balls, rpms or git repos. It is pretty easy to create a PKGBUILD file, build a package and install the software with the package manager. This is a solid competitive advantage from my point of view.