Starcraft ranks #10. It was released in 1998 and is still popular, pitting the Zerg, Protoss, and Terran against each other with unique play styles.
SiN Episodes: Emergence ranks #9. It was the first game created by a major developer released without a publisher on Steam. It is a FPS that takes place in a future mega city where you play a cop injected with an experimental drug seeking a cure.
Sins of a Solar Empire ranks #8. It is a popular space RTS released in 2008 with expansions, where you colonize planets through force, money, or religion as one of three factions. You can zoom from ship to galaxy views.