Mostly, I tend to put the RTS' into ideas about "real-time strategy."

Dune II: One of the first, one of the best. 3 factions, all very different, story mode with scope. Interesting units when "harvesters" was a new idea. https://en.wikipedia.org/wiki/Dune_II

Command & Conquer / Red Alert: Excellent unit select, very orthogonal unit choices, live-action cutscenes (which while of questionable quality, were still pretty amusing) Red Alert also has an excellent soundtrack (although extremely facist march themed with "Hell March", yet quite listenable). https://en.wikipedia.org/wiki/Command_%26_Conquer:_Red_Alert

Age of Empires: Artificial Intelligent opponents that didn't "cheat" with extra resources or out-of-game knowledge, they were simply competent opponents. Age of Kings is probably the showcase. Rotational rock-paper-scissors unit effectiveness. Several "victory" types. Gameplay that tended to support longer games, rather than 3-minute rush (Commander vs Legacy in an MtG context). Only complaint is unique units were often not enough for long enough to really be special. https://en.wikipedia.org/wiki/Age_of_Empires_II

Warcraft II: Not sure how relevant today, yet one of the first to feature an online matchmaking service that was not utter misery (and actually had players at lot of the time!) Also, neat cartoony style that became the basis of WoW, DOTA, and many others. Possibly the best though, first game where you could spend half an hour just cycling through all the quotes ("I can see my house!") for units (and their annoyances at your clicking "Are you still touching me?"). https://en.wikipedia.org/wiki/Warcraft_II:_Tides_of_Darkness

Homeworld: There is Still not a significant other 3D RTS franchise (EVE Online is very close for an MMO). Basically, about the only game that does actually 3D strategic space combat in an actual spherical playing field over vast distances, with multiple-ship scales, and per unit positionable cameras ("camera can be set to follow any ship and view them from any angle, as well as display the ship's point of view"). The story's also quite well written, and at least provides a plausible reason for each of the missions. I watched Star Wars: The Last Jedi, and thought "You people should have played Homeworld, you would have written such better space battles. Somebody over at Battlestar Galactica must have played Homeworld." https://en.wikipedia.org/wiki/Homeworld

Sins of a Solar Empire: Neat game design based on solar system "zones", where you can only really fight around solar systems, and then transit is mostly hands off (often with surprises when you exit jump space into fog of war). Planetary and solar system upgrades that are inherent to the idea of "planet" rather than just tower defense. Multiple ship styles and races with fairly different play styles depending on the build choice. Been a while, yet from what I remember, also not quite as punishing as Starcraft about choosing the "correct" choice. https://en.wikipedia.org/wiki/Sins_of_a_Solar_Empire

Warhammer 40K Dawn of War: Gameplay that's not built on mining anything, rather more like capture the flag, holding locations for time frames to accumulate "something". And then pretty much all the pre-existing craziness of Warhammer that is far too long for a single game recommendation. LOTS of units and armies though. https://en.wikipedia.org/wiki/Warhammer_40,000:_Dawn_of_War

Total War Franchise: (Medieval II, Rome, Shogun, and Warhammer are notable entries) Scope. Massive scope. Battles with 10's of 1000's of soldiers on the field. Became a bit much to deal with in some of the games, yet neat to be able to create your own LotR "Ride of the Rohan" scenes. https://en.wikipedia.org/wiki/Total_War_(video_game_series)

Achron (have not played, would like to try) Time travel RTS with three time-frames units fight in, and propagating time waves that erase the future. You can "chronofrag" yourself, by having future self meet past self. A Grandfather Paradox resolution system, where units are removed if they fall out of the "possible futures."

Supreme Commander: (and prior Total Annihilation) are the subject of this article. Also massive scope. Huge, smoothly scalable maps. Tons of unit choices. Enormous numbers of on map units simultaneously.