Id have to say the PSU,(Power supply unit) You could have the best rig ever but if you dont have the powersupply to run it it you cant run it to its maxium potential.."A psu is to a computer like a heart is to a human"..I cant rember who said that but they are **** right
i would have to say RAM, CPU, and video card put together. if you dont have a powerful CPU and alot of RAM you cant run good games. and if your card dosent support some features of the game then you cant get best quality or even run the game.
i think is the video card and ram with cpu coming last. I f u want the best graphics u ge the best GPU and the enough ram. If u want to crunch numbers like video editing or music conversion then a powerful cpu.