For a single battery mod, a 21700 or a mod like the Vaporesso Nebula that takes a single 26650 (still light, LONG battery life, compact size). These make good carry around size mods, lower power vaping but enough battery capacity to last all day.
Otherwise dual 18650 for home use, higher power. I run all of my mods conservatively, so the extra power is more about safety for me than it is about battery life.