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.