I’ve thought about it and Chemists are by far the best specialization for something of this nature. Being able to bring back semi-modern metallurgical techniques would instantly rocket you to the 18th century. Even if you couldn’t do that, the modern atomic model and electron would be huge. And thousands of years of advancements in mathematics. Then of course you have some basic reactions and chemical processes, for example the haber-bosch process (for fertilizers). Then you have other random things like refrigeration, bombs, batteries, light bulbs, matches, thermometers, plastics (!!!), etc.
Depending on what you know, you could even build rudimentary photovoltaics, combustion engines, oil refineries, radios, practically anything.
The biggest thing that any scientist could bring, however, is a scientific revolution. Getting people to adopt the same mindset of discovery and empirical observation should be the greatest advancement you could bring.
Anyways, there are definitely a few key things any average Joe could “invent” if he were transported to the past:
>microscopes and telescopes