[mhh__]

RISC-V probably has the most "serious" effort of anything other than ARM and X86.

[datameta]

That's a fair estimate, but let's not forget IBM's Power architecture.

[mhh__]

Power is very cool and I want a Power10 box a lot but I'm not sure if it's fair to really decide in it's favour because POWER is basically unobtainable whereas you can actually buy dev boards for RISC-V from SiFive.

[afr0ck]

There is also Sparc and to a lesser extent MIPS. They are more mature than RISC-V and have a decent software ecosystem.