It's not a research paper, but for learning purposes there's nothing better than the Physically-Based Rendering (PBRT) book by Matt Pharr, Greg Humphreys and Wenzel Jakob, already mentioned by others in this thread: http://www.pbrt.org/
One of the foundational paper is definitely The Rendering Equation, by James Kajiya: https://inst.eecs.berkeley.edu/~cs294-13/fa09/lectures/p143-...
Another highly influencial research paper is certainly Eric Veach's PhD thesis, Robust Monte Carlo Methods For Light Transport Simulation (a.k.a. "The Bible"): http://graphics.stanford.edu/papers/veach_thesis/thesis.pdf
A few other paper stand out:
Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs, by Eric Heitz: http://jcgt.org/published/0003/02/03/. A somewhat difficult read, but an important paper.
Microfacet Models for Refraction through Rough Surfaces: http://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.pdf
Physically-Based Shading at Disney: https://disney-animation.s3.amazonaws.com/library/s2012_pbs_...
A Practical Model for Subsurface Light Transport, by Henrik Wann Jensen: https://graphics.stanford.edu/papers/bssrdf/bssrdf.pdf
Light Transport Simulation with Vertex Connection and Merging: http://cgg.mff.cuni.cz/~jaroslav/papers/2012-vcm/
Finally, for learning, there's a nice lecture by John Carmack at QuakeCon: https://www.youtube.com/watch?v=IyUgHPs86XM