Trading Group Theory for Randomness by Laci Babai (http://dl.acm.org/citation.cfm?id=22192) -- this beautiful paper introduced algorithmic group theory & interactive proofs (in the form of Arthur-Merlin games) to study the Graph Isomorphism problem, and introduced several groundbreaking new results. Perhaps a more approachable (and funny) version of this would be Babai's humorous essay detailing the flurry of work that broke out after his results introducing AM/MA...it's the closest thing I've seen to making theoretical CS exhilarating :P (http://www.cs.princeton.edu/courses/archive/spr09/cos522/Bab...)