restic without any doubt. I use it with S3 backend and SSH copy and it has an excellent performance (with copies of years).

Borg I was using it for a while (to compare) and I do not recommend it, it is not a bad product, but it has a lousy performance compared to restic.

Kopia I didn’t know it, but from what I have read about it it seems to be very similar to restic but with some additions to make it pretty (like having ui).

Some people say that Kopia is faster in sending data to the repository (and other people say it’s restic), I think that, unless you need ui, I would use restic.