I personally have a soft spot for StarCraft2. Being the First RTS I really got into and played. The fact that there is still awesome modded content released in game and from external mods so many years later is amazing to me. I am happy to see that the genre is still getting love in the form of love for old games and new stuff. I wanna get into more at some point when life isn't so overwhelming, so tell me what has your experiences with RTS games has been like :3
P.S. If you wanna play StarCraft2 Co-op I am bored and the game is Free :3
For your traditional RTS I probably have to go with Dawn of War. But if we're also including grand strategy RTS it's Stellaris. It's insane how much content there is for this game. It's the be-all end-all of "space games". If you can think of any sci-fi trope at all, there's content for it in stellaris.