My mention goes to V. I Lomonosov's "Invariant subspaces for the family of operators which commute with a completely continuous operator", Funct. Anal. Appl. 7 (1973) 213-214, which in less than two pages demolished numerous previous results in invariant subspace theory, many of which previously took dozens of pages to prove. It also kick-started the theory of subspaces simultaneously invariant under several operators, where it continues to be useful today. It's highly self-contained, using only the Schauder-Tychonoff theorem, if I remember correctly.