Among my favorite Severin works:
1. Western and War short stories for Warren in the late 60s and 70s.
2. His run on Our Fighting Forces with Bob Kanigher, from around issue 135-150 (just before Kirby.
3. As Lawson mentioned, Blazing Combat.
4. Kull the Conqueror (he inked his sister Marie).
He returned to comics with a vengeance this decade, even though he was in his 80s. He drew Rawhide Kid, Desperadoes, Caper (issues 5-8 of a Judd WInick mini-series), Punisher Max (a one shot called the Tyger, written by Garth Ennis), Bat Lash, and (as Allan said) Witchfinder.
Every piece of Severin artwork I ever saw was nothing short of terrific. He was truly one of the master illustrators of comics.
RIP John, and thanks so much for always giving us high quality work.