I'm tempted to say plastics, but I'm also actually kind of optimistic that either we'll engineer fungi / bacteria to decompose it, or said fungi / bacteria will evolve on its own to decompose it. We're already seeing signs of the latter.

https://www.theguardian.com/environment/2021/dec/14/bugs-acr...

For that reason my vote is on processed foods as well as foods grown "industrially", for the simple fact that we can't really support 8+ billion people without them. On an individual level, sure, you can eat whole foods and grow your own in rich soil that yields nutritionally dense vegetables if you have the means, but on a systemic level I'm not sure you can undo all the changes in our "food system" without also causing a global famine. Besides, are you really going too miss-out on a night with friends at a restaurant because they use seed oils? Is that same restaurant willing to increase their prices in order to cover the cost of using good ingredients?

So yeah, I think for now we're going to have to contend with nutritionally-poor vegetables, pesticides, herbicides, novel zoonotic diseases from animals living in cramped and unhygenic virus factories (the next pandemic is more likely to come from a pig or a chicken than a bat or a pangolin), and plant-based meat which a complex list of ingredients that most people are too lazy to personally vet.