I remember using the Minix filesystem on floppies. ext{,2} was a bit too heavyweight for a floppy. And UMSDOS on my MS-DOS6.22/Wfw3.11 FAT16 partition. And HPFS so I could mount my OS/2 2.0 partition (EMX on the Hobbes OS/2 CD-ROM was my gateway drug to Linux.)

xiafs sounded intriguing, but I never actually used it.

Of all the filesystems currently in the mainline Linux kernel, the most useless (surely) must be BFS (not BeFS) – https://www.kernel.org/doc/html/next/filesystems/bfs.html