Which file format do you prefer? And why ?
ext4, xfs, btrfs…
Highly subjective query. 
For myself I use a combo BTRFS and ZFS, on different systems.
For the family I format it as ext4
Because I can, they work nicely with my back-up routine.
Depends on the system setup. For snapper usage Btrfs (most tested due to being the original format when snapper was introduced). For a non snapper setup ext4.
BTRFS, if anything breaks, I can just go back and fix it. Besides, the file format performance difference is very minimal.
I prefer Btrfs. Never had a problem with it since it was introduced. That’s the main reason why I chose this distribution. Plus it’s always nice to have snapshots as a safety net. I have a separate SDD in my computer as a storage backup for the important stuff.
.txt? 
.. but no, we were talking about filesystems and not file formats.
Title edited accordingly.
BcacheFS - nice Mix Data Secure + Speed with a lot outstanding Features no other FS has suddenly. I am using it for 1 year since never had any Dataloss 
ext4, I do not use snapshots and I like the reliability record of it.
Btrfs with snapper for ease of use just in case. If it was a less leading edge distro, I would use ext4
Might be in the minority here but since I started using xfs I haven’t looked back. Obviously it lacks the shiny new features but it’s been incredibly stable and incredibly fast. No issues whatsoever. Snapshots/Backups are still easy using rsync/BorgBackup.
For desktops: BTRFS, because it has great features: snapshots (for data safety, it doesn’t replace the need for backups, but in most cases you can restore data very quick), Copy On Write (SSDs does not wear out as quick) and compression (so you can store more data and it also let your SSDs live longer).
For database servers: XFS, because it is very quick at parallel reads and writes. It also works great as a local cache drive for NFS mounts with cachefilesd.
If I would have an webserver, then I probably would use ZFS because of the RAM cache. Also very nice for a NFS server or a NAS. It features snapshots, COW and compression like BTRFS, but as an out of kernel filesystem it’s not a thing I would use on a roling release distribution like CachyOS.
Ext4 is backwards compatible with Ext3 and Ext2, nice for transfering stuff to very old linux systems, but that’s all about it. I wouldn’t use it for a new linux installation nowadays.
Can you explain more about BcacheFS? Is it any good comparing to Btrfs on desktop? Why do you prefer it? Just because of “outstandness”?
Since I started using Linux, I have been formatting with ext4.
As I have never had any problems with it, I will stick with it if possible.
For internal drives (SSDs) I only use BTRFS. I love snapper snapshots and if I don’t like an update because it behaves weird I recover my old system in no time.
For external drives, almost USB-drives for backups I prefer ext4.
Longtime Ext4 user. But probably going to finally swap it out for XFS in the near future.
I have been saying an overdue format is imminent for … what feels like months.
Ext 4, it just works.
I prefer zfs over all others. It seems as btrfs has a good feature-set also, but reading about the long way with issues then can corrupt your days I can’t trust it throughout.
Sadly zfs as roofs in Linux is not always fun, as it is often not on par with the kernel and boot environments are only supported by zbm which is mostly no choice in the installer.
So for my notebooks I go btrfs, of course with regular backups.
I prefer OpenZFS with a LTS kernel. Staying LTS make sure you don’t get a new kernel that doesn’t support ZFS yet. ZFS is way more reliable and solid than BTRFS and has good rescue tools. If you want to stay simple EXT4 is a good choice too.
Btrfs on CachyOS. I’ve been really happy with it. On my servers I mostly use ZFS but have some EXT4 drives.
I’ve got Backrest running Restic on my CachyOS pc, sending it to my Proxmox Backup Server which has a RAIDZ2 pool. Next step is to get some cloud storage going. Probably end up with Backblaze but I want to tune my backups a bit first so I’m not pushing extra data.
How are you sending restic to PBS?
Currently I am using restic on my clients and PBS to save my PVE.
I was planning to switch from restic to proxmox-backup-client, but if there is a way so push restic directly to PBS this would be great