Galactic Patrol
The heart of the foundational space opera series; the Lens is a direct ancestor of the Green Lantern Corps, and the Boskone war shaped all galactic-conflict SF after it. (Within-series numbering counts the later prequels.)
Series Guide
Lensman
Doc Smith's escalating cosmic arms race between Civilisation and Boskone, secretly a two-billion-year war between the Arisians and the Eddorians. The original widescreen space opera.
In the Guide
Gray Lensman
Often cited as the series' peak and a defining document of pre-war super-science SF; its arms-race structure echoes through military SF to this day.