Einstein, Albert. "On the electrodynamics of moving bodies." Annalen der Physik 17.891 (1905): 50.

http://ganapathymani.com/On%20the%20electrodymics%20of%20mov...

This paper establishes special relativity, and is remarquable for how clear it is, revolutionizing physics while using only elementary math. The first "Kinematical" part in particular does not use anything more complex mathematically than Pythagorus theorem. It is so clear that the explanations and though experiments are reproduced in all textbooks to this day; the only change is that textbooks include diagrams.