Knowing what will have the biggest impact on your image quality is hard, as it varies so much from genre to genre. For still life and food photography, this is the order of importance I have come up with.
This video discusses how to prioritize spending money when you are ready to begin upgrading lights, modifiers, cameras, and lenses. Although there are lots of throwaway sayings about investing all of your cash into lenses that get banded about on the internet, this isn't always the best way to go. By breaking down the genre I work in, I was able to find the best areas to spend my hard-earned cash on.
I will start this off with a small disclaimer: I shot on crop sensor cameras with cheap lights and modifiers for a good six years. I still got paid and made a living from photography, and some of those images are still in my portfolio. It took me over a decade to collect all of the gear that I currently have, and I did so one item at a time. There is, of course, a huge amount I still want to upgrade, and I use the method discussed in the video to decide where to put my money.
There are loads of sites and channels that offer advice on technical image quality when it comes to cameras and lenses; however, there is much more to how good an image looks than sharpness and dynamic range. A lot of what these websites don't and can't discuss comes down to aesthetics and taste. The more important metrics are qualitative rather than quantitative for a lot of genres of photography. Working as a commercial food and still life photographer really brings this to light.
How do you prioritize your new photography purchases in your genre?