I would choose an intervention that would increase the quality of life for the most number of people possible. With that constraint, the intervention should be easily reproduced and inexpensive. It should also minimize the risk of being used incorrectly, with possibly unforeseen negative consequences.

That is why I would bring inexpensive microscopes and information on germ theory, with the aim to introduce hand washing and food hygiene practicing to as many regions of the world as possible. In this scenario, bringing an effective campaign to disiminate the information (pre-printed, with infographics) is more important than the technology itself. 
 

I’m unsure where in time exactly would be the best point to do this. Perhaps sometime after world travel via boats is reasonable to do in a lifetime.