TITLE: Which of Quillen's Papers Should I read?
QUESTION [52 upvotes]: I just heard that Daniel 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 geometer 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.
Here 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.

REPLY [13 votes]: Quillen's book on Homotopical Algebra is a great pleasure to read, and likely to appeal to a geometer.

REPLY [11 votes]: There are a number of papers that I have affection for.  Those which I don't see listed:
"On the group completion of a simplicial monoid."  Before I read this paper I never really understood the appearance of the plus-construction.  This essentially proves the +=Q theorem.  (A little harder to find: Appendix Q in Friedlander-Mazur's "Filtrations on the homology of algebraic varieties.")
"On the formal group laws of unoriented and complex cobordism theory."  A very influential paper, along the likes that Mark Grant mentioned, and one which we've been trying to unravel the consequences of ever since.   (Bull. Amer. Math. Soc. 75 1969, 1293–1298.)
"The Adams conjecture."  Quillen's proof of the Adams conjecture by making use of a Brauer lift is short but wonderful.  (Topology 10 1971 67–80.)