For me, the crown has to go to the Command & Conquer series, with Red Alert being the absolute highlight. The blend of campy storytelling, fast-paced base-building, and satisfying unit combat made it genuinely addictive in a way few RTS games have managed to replicate. Red Alert in particular struck a perfect balance — accessible enough to pick up quickly, but deep enough to keep you coming back.
The original Command & Conquer deserves equal credit for laying the groundwork that defined the genre. It introduced the resource-harvesting, base-building formula that so many later titles borrowed from, and its FMV cutscenes gave it a personality that still feels iconic today.
Honorable mention to Red Alert 2 as well — arguably the most polished entry in the series and one of the most fun RTS experiences ever made.