Ext2 on OpenBSD is good enough, in my opinion. Although it's not really battle-tested by a lot of users, I've been sharing /home between OpenBSD and Debian for >1 year on ext2 with no problems (lots and lots of reads and writes of many files), but OpenBSD's fsck_ext2fs is dodgy, don't use it. If the other BSDs ship with ext2 tools, they're probably similarly untested and generally unused. I wrote an article about it too, but that's the general idea.

The support on Net and FreeBSD must be better or at least as good, but I don't really know, someone else would have to chime in on that.

If you involve Windows in this, every ext2 driver I tried required a long e2fsck session from Debian to fix it after a write from Windows, so that's likely worse than useless. But you didn't mention Windows, so this is perhaps not relevant.