The Most Highly Cited Paper in Publishing History: Protein Determination by Oliver H. Lowry
Protein Measurement with the Folin Phenol Reagent (Lowry, O. H., Rosebrough, N. J., Farr, A. L., and Randall, R. J. (1951) J. Biol. Chem.193, 265–275)
Hasnain says:
"As of January 2004, it was cited 275,669 times."
that's just insane.