That's a good question. Probably either RISC-V or ARM. MIPS is interesting, but I'm not sure that it has much of a future. OTOH, there's plenty of choice and availability of both ARM and RISC-V based computers to play around with. RISC-V is newer and probably still evolving more rapidly, so I guess you'd want to weigh whether you consider that a pro or a con.
For me personally, I have recently gotten really interested in RISC-V, so I lean that way a little bit. But that's more personal preference than anything concrete I can really cite.
That's fair. But there's still changes going on in terms of extensions, and then there's the implementation level stuff (both hardware and software). So I guess I was really thinking more in terms of the overall ecosystem than just the instruction set.
For me personally, I have recently gotten really interested in RISC-V, so I lean that way a little bit. But that's more personal preference than anything concrete I can really cite.