I suppose we differ in our definitions. Which is absolutely fine, to be honest*.
For completeness' sake, IMO it's basically the intersection of Major Distributions and Independent Distributions. Which happens to consist of Arch, Debian, Fedora, Gentoo, openSUSE and Slackware.
Out of these, Arch and Gentoo don't have defaults, but their documentation uses ext4 most frequently for examples. For the remaining four, Fedora and openSUSE default to Btrfs. While Debian and Slackware default to ext4.
In all fairness, one might argue that Distrowatch's list of major distros is arbitrary. Therefore, we could refine what's found above by including actually data. For this, I'll use Boiling Steam's usage chart based on ProtonDB's data. This ain't perfect either, but it's the best I can do. Here, we notice how both Gentoo and Slackware are not represented. Furthermore, NixOS poses as a candidate instead. For which, we find that (if anything) ext4 is the default. Regardless, it doesn't actually impact the earlier outcome:
- Arch (and Gentoo) don't have defaults
- Debian(, Slackware and NixOS) default to ext4
- Fedora and openSUSE default to Btrfs
Anyhow, what are the main distros according to you? Please offer an exhaustive list, please. Thanks in advance!