Unix is not a monolithic system controlled by a single entity. So this is harder question than you may think.

The original filesystem for unix is one that was designed by Ken Thompson back in the 1960's. One one his reason to write unix was to try out his filesystem. It didn't really have a name that I know of. While it was very cool for its time, it was pretty much abandoned by 1980. It had some serious problems. I doubt that anyone would want to use when alternatives became available.

The next major filesystem was designed by Kirk Mckusick. It is very good and it is still with us today. HP-UX calls it "hfs" for high-performance filesystem. SunOS calls it ufs. I don't know what the u stands for. But even though both OS's use this filesystem, you can't umount a disk from HP-UX and carry it to a SunOS box and mount it. There are enough differences that this won't work.

Both SunOS and HP-UX have other filesystem types built-in. Both are moving to the new Veritas filesystem and both call it vxfs. But again, you can't cross-mount a physical disk. Both OS's also have support for cd-rom type filesystems. And they both support NFS which let's you open a file on a remote system.

At this point, Unix has special features in the OS that allow easy installation of new filesystems. It is almost as easy as installing a device driver. Writing a decent filesyetm is harder than writing a decent driver, so there aren't lots of choices. But are several odd filesystems that float in from time to time. HP-UX invented its own which it called SDF, for "Structured Directory Format"...it didn't last. I have also heard of AFS (Andrew Filesystem), JFS (Journeled Filesystem), and few others that I recall just this minute.

So it's not easy to give you a definative answer.