I just heard that Dan Quillen passed on.  I am not familiar with his work
and want to celebrate his life by reading some of his papers.  Which one(s?)
should I read?

I am an algebraic geometry who is comfortable with cohomological methods in his field, but knows almost nothing about homotopy theory.  My goal is to gain a better appreciation of Quillen's work, 
not to advance my own research.

I tagged this question as "at.algebraic-topology, algebraic-k-theory" because I think these are the main fields in which Quillen worked.  Please add or change this if other tags are appropriate.