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The rockets and the gun. I can use the rockets to prove I have useful knowledge while using the gun to prove that my stuff is mechanisms not magic (by taking it apart). I'll then set about teaching them how to make gunpowder (charcoal, sulphur, saltpeter) and once I'm truly established we can move on to basic firearms.
You might want to work on making quality steel first. Gunpowder will be on its way from China already, so the time will be better spent on some critical materials.
Since you will need funding for other major projects, like smokeless powder, you might be better served by outfitting a few armies with better armor and swords first. (Take advantage of incremental science improvements, basically.)
Even with your vast experience with magic technologies, it's still going to take a fortune to develop the basics. A small research center is going to cost you at least a few hundred cows, a couple thousand chickens, and a few dozen horses, after all.
(You will need steel for mining operations to get enough raw materials to do anything, actually.)
And also a degree in metallurgy.
I guess it depends where and when you are in the medieval era. In some places even knowing that steels properties are largely determined by its carbon content and not some other component of the regional addatives would be a valuble insight. In other places they already have metalurgy developed enough for spring steel so even with a degree it might be hard to implement improvements with the other technology available.