J.-C. Yoccoz called
Carl L. Siegel, Iteration of analytic functions, Ann. of Math. 43(2) (1942), 607–612.
a "brief but historic article". In only 6 pages (including all necessary background) Siegel gave the first positive solution to a small denominator problem. This had been a major unsolved issue for over 60 years, and was a big thorn in the side for Poincaré. Siegel's paper is also credited with inspiring Kolmogorov to start the circle of ideas that led to KAM Theory. Buff, Henriksen, and Hubbard did not hesitate in calling it “one of the landmark papers of the twentieth century.”