There is no "best" upgrade for everyone. It all depends on what you use your computer for. It also depends on the age of your device that will determine if an upgrade will have any effect on performance. We can determine if upgrading a component or two would be performance boost or cost prohibitive.

If you do a lot of multitasking, or your work involves applications that require a lot of memory (e.g., you run a lot of virtual machines), RAM is going to be a solid upgrade.

If you play a lot of games, a new video card is more likely to increase performance.

Video creators/editors would probably like a faster, multithreaded processor.

And maybe you just Web Surf and check your e-mail, maybe upload and share a photo now and then and your PC is just slooooowwww!   Replacing your old legacy hard drive with a much faster Solid State Drive, will add a few more years of useful life to your device.