ZFS is great on servers with lots of ram, lots of cpu, and lots of disks.
It does very well on large disks and joining multiple disks, and has nice features like software implementation of hybrid spinning rust + SSD cache raid volumes for extra performance.
If you have a single small disk with limited ram, zfs might still work well, but most of its advantages are gone, and ext4 might work as well, but with lower ram use.
For single disks over 4T, I would consider xfs over zfs or ext4. For more than 3 disks, or a spinning disk with ssd, zfs starts to look very interesting. Between 2T and 4T on a single disk, any of these would probably have similar performance.
But, as always, your specific use case affects this greatly, and there are corner cases where any of these would work better than the others. For example, zfs and xfs perform much better than ext4 when you have thousands of files in a single directory, no matter what the disk size. But this is a corner case, and not common, especially with disks smaller than 2T.
For small disks typically found in laptops, ext4 should be sufficient, and if you want encryption, the defaults you get when you enable it in the ubuntu installer probably work best with ext4 anyway.
