most reliable operating systems tend to be mission-critical OSes that a lot of people arent familiar with, like RODOS, an
operating system designed for satellites and space probes, and VxWorks,
the operating system NASA uses for the Mars Pathfinder and Curiosity
with three primary features: reliability, reliability, and reliability.
Every line is hand-traced and hand-checked multiple times, so as you can
imagine they’re very expensive to develop.
also incredibly robust, designed not only for reliability but also to
tolerate things that bring most other OSes to their knees, such as
memory corruption or other hardware failures.
next most reliable operating systems are mainframe operating systems
like z/OS (IBM’s operating system for their z Systems mainframe) and COS
(a batchmode operating system for Cray supercomputers).