It depends on the budget and needs. For someone who wants a computer cheap as chips for office use, the priority would be on finding a motherboard with integrated graphics, and building from there. For someone who wants to play games at high settings and is willing to stretch to higher budgets, the graphics card is most important, followed by the CPU. Then you need a power supply that's strong enough and a motherboard to put them all together. The rest is just filling in the gaps: case, DVD drive, HDD, cooling, operating system, and peripherals.