Developing an OS from scratch by one person that competes with iOS, OSX, and Blackberry, or developing a series of hardware devices (allso from scratch) to program for, i.e., a motherboard, CPU, VPU, I/O, PIC, PIT, IPC, sound chip, etc.
*(A bit off topic)* I want to create computer motherboards. My tech teacher is a CS, but I am too shy to let him know I am a programmer...
Anyway, I'm guessing creating your own hardware. I glanced at ASM tutorials, but I'm guessing making your own CPU/GPU/Ethernet Board would be hella hard to do.(I don't have the patients to create my own custom motherboard out of XOR, OR, NOT, AND, etc. switches D;
I think making a motherboard would be harder, unless of course, you have machines that can make nanoscale circuits and electrical components, and a room 100x more sterile and cleaner than a hospital OR.