I recommend Assembly, specifically for classic 8-bit CPUs like the Z80 or 6502. Assembly is unique and fun and, if you go with the classics, the learning curve

I recommend Assembly, specifically for classic 8-bit CPUs like the Z80 or 6502. Assembly is unique and fun and, if you go with the classics, the learning curve is shallower and you won't be distracted by the constraints of modern chips such as pipelining and power consumption.