4 : the CPU, which is #1.
When you buy a computer, that's what you're looking for. Everything else is easy to upgrade or, if labeled correctly as a gamer, it already comes with the best other stuff possible.
Now 2022, in 2014 GPUs were about to go nuts and have since trapped any CPU that isn't absolutely top notch as the bottleneck. That you're going to have an SSD and the best ram that fits is a given or it isn't a game box. Also, you're spending over $1k just to get 30fps or less at 4k on ultra. If you want 4k at 60hz, then you're spending $3k or more. If you're not into 4k on ultra, then these days anything will work.
Spend all your money on the CPU. Don't put a $600 GPU in a $300 computer and expect it to do stuff. I always saw marginal performance increases when upgrading video cards. But then I went from an ancient i7 quad core to a hyper-threaded Ryzen sexta-core with 14nm technology. It's more than twice as good, with only half again as many cores, at 4k Hz instead of 3.5k Hz.
Hyper threaded (always, because why teh eff not?) As many cores as you can afford. Get the smallest chip spacing available (e.g., "14nm") with the highest Hz you can afford (both base speed and OC'ed). Everything else follows.
If you spend enough money on a prebuilt box, if it comes with a monster of a processor, it's probably got sufficient other stuff. But saving money by being cheap on the CPU, to later have some for upgrading anything else, is silly.
I spent ~$1k on mine. There's nothing that needs to be done to it. And my only buyer's remorse is that there's nothing that can be done to it. Oh well, I guess I'll just go play Fallout 4 (modded, on super-duper) ultra, at 4k, at my measly +/-30fps.... My only true regret is that it caps out with 2666MHz DDR4 RAM.
A modern i7 is better than a Ryzen 7 by 15~20%. But it's also twice as expensive (~$300 vs $600). And when it comes in a prebuilt, more like triple.