Linux seems to be able to deal with UFS2, an advantage over FFS and UFS1.
ZFS is realy not an alternative, I have only 250 MB Ram and 800 MHZ geode
processor.
With 2 FS I have not always the best experience, but is perhaps the most portable.
There is a lot of file systems, but it is still difficult to decide what to use.
Most Linux distributions (including Arch, Debian, Ubuntu) have read support for UFS but write is disabled in the kernel. You have to build a custom kernel to use the write capability, which is definitely not convenient. Some distributions (including openSUSE) have it enabled in their default kernel, but still, if you want your stick to be usable on most computers (including Windows and MacOS ones), don't use UFS, neither ZFS.
FAT (msdosfs) is probably the worst file system still used today, but (and because) basically any OS can read/write it out of the box.
exFAT is a somewhat better replacement for FAT, which is also natively supported on Windows, MacOS and Linux (5.4+) but needs a FUSE module from ports to work on FreeBSD.
Some people use NTFS for that purpose, it can work on most systems, it is journaled (can be both an advantage and a disadvantage) but still requires FUSE on anything other than Windows.
Since I use USB sticks mostly for transferring files from a computer to another, which often have different operating systems, and I don't want journaling, all of mine are formatted with exFAT.