What are the skills and knowledge you could actually bring & fully realize at some point in the past?
And we're taking this in the strictest, nerdiest, materialist lense. I don't care how smart you are you ain't making a steam engine the in bronze age, for instance.
So what could you create, with just your knowledge & period tools? What kind of institutional, technological, philosophical innovations could you realistically recreate? How would you interface with the social fabric of society to not be some crazed pariah who never positively influences the place they went?
Inoculation is an easy one if I could get someone to believe it, as well as whatever basic medical knowledge I have on hand (cool it with the bloodletting maybe?). Might be able to make a simple battery if I thought hard enough.
Could do a crude musket if I could figure out gunpowder (can remember saltpeter and ash(?) off the top of my head). That opens up cannons too. Assuming I've traveled to before these were readily available, whoever I swear allegiance to could have access to briefly unmatched firepower until the technology was copied. Crossbow should be possible as well, and also doesn't need as valuable of resources
Outside of the 20th and 21st century, I think most things should be replicable pretty far back, like into the classical age if you're somewhere fairly "developed" like Rome. It's hard to underestimate the importance of simply knowing that something is possible because it was done before.
gotta have some metallurgy to make gunpowder give you muskets though I think