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.
Armored Brigade and Call To Arms Warfighter mod I guess are slot two and three.
I haven't tried Supreme Commander Forged Alliance but I enjoy watching games (I enjoy watching BAR as well).
I find C&C and Starcraft-esque RTS games boring and stiff though, I have a lot of issues with where the genre went and I am not surprised it became a relatively forgotten one. The rock paper shotgun simplistic unit relationship micro heavy gameplay feels unsatisfying like the combat in a MMORPG compared to more realistic wargames or a Total Annihilation-like such as BAR or FAF.
edit I realize there is obviously a lot of skill and depth to Starcraft, I just am bitter the world went with the Starcraft model and basically totally ignored Total Annihilation for a decade or so lol.