ext4
Default Linux filesystem for most distros (Ubuntu, Debian, Arch). Successor to ext3 (2008). Extent-based allocation, journaling, max-file 16TB, max-fs 1EB. Stable, well-tested, performant.
Classic choice: good default that "just works". Distinct from XFS: ext4 is better for small files + mixed workloads, XFS better for large files + high concurrency. Distinct from Btrfs/ZFS: no native snapshot, no checksumming, but also no extra RAM overhead. Modern improvement: project quotas (since 3.16), encryption (fscrypt since 4.1), case-insensitive directories (5.2). Cloud default: AWS EC2 default ext4 on Ubuntu AMIs. Good default recommendation: ext4 if you don't have specific reason for XFS/Btrfs/ZFS.