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?
Maybe some math. People were really interested in tracking movement stars, planets etc. So if you learned their system of notation you might be able to speed up the development of certain mathematics since they’d see the "practical " value in it for astrological or religious purposes.
Edit: just realized Mark Twain’s Connecticut Yankee is pretty much an answer to this question. Not sure it holds up to materialist analysis though.