Check out this list from EFYTimes that lists, in their opinion, the 5 Best Raspberry Pi Operating systems of all time:
Raspbian is a free operating system based on Debian optimised for the Raspberry Pi hardware. An operating system is the set of basic programs and utilities that make your Raspberry Pi run. However, Raspbian provides more than a pure OS: it comes with over 35,000 packages, pre-compiled software bundled in a nice format for easy installation on your Raspberry Pi.
The initial build of over 35,000 Raspbian packages, optimised for best performance on the Raspberry Pi, was completed in June of 2012. However, Raspbian is still under active development with an emphasis on improving the stability and performance of as many Debian packages as possible.
2. Risc OS
RISC OS is a computer operating system designed in Cambridge, England by Acorn. First released in 1987, its origins can be traced back to the original team that developed the ARM microprocessor. RISC OS is owned by Castle Technology Ltd. Risc OS Open Limited (ROOL) manages the source code for this operating system.