Power supply of course is the main most crucial, but the cpu/motherboard/memory/harddrive/heatsink/graphics chip are just as important, as you cant really have a computer without them. If your talking about which one is the most important to get something quality, then the power supply tops that as well, as cheap ones have been known to destroy other components. The reason i have my sig the way it is is simple, i have seen way to many instances where power supplies have failed on people, killed a motherboard or gpu, and they yell at the graphics card or motherboard manufacturer. Plus through the life of a power supply, even if you replace a $30 one once, thats $60 that could have been used on a quality one to begin with. I wish my sig had more room...
FYI, nobody is gonna change their sig because of this thread. Well, i reworked mine so it read a bit better after seeing this but other than that..
Everything is important in a computer, except maybe the optical drive... The most crucial to get something quality though would have to be power supply for obvious reasons, and hard drive because obviously you dont want to lose data.