If any of us has heard of it, then it is not obscure enough to be the least successful OS.
I would guess maybe heathkit's HDOS since users had to build their computers and few could afford the disk system, if not then you would have to go back to the proprietary OS's of the mid '70s to early '80s.
How about Forth which was meant to be an operating system, wasn't it?
How successful was TRIPOS before it was used as the basis for Amigaos?
