Sacrifice is a good game here on gog.
I would also like to mention the Myth series (I and II mainly) which are RTS games from Bungie (before they moved to consoles/halo). One of the best RTS games imo. However sadly it's not available on gog or any digial distribution site IIRC, you'll have to go the ole physical box route on ebay or amazon, as the IP is tied up with Take 2 after bungie sold it off to another dev who then got shut down and it defaulted to take2, who seems content with doing nothing with it, no new games or interest in it at all.
The games, unlike most other RTS, do not focus on base building nor resource gathering, rather it's purely about the storyline and combat. Each mission unfolds part of the storyline as you play it and you are given a set number of troops to command and that's that, you have to make the best use of them you can and get to the end with troops left. Sometimes you can get reinforcements (for story related purposes) but that's about it.
The combat is one of the best in any RTS games. Evelation, weather, all play a part in combat. Archers and your range units all get longre/better range at higher evelations, rain/snow/water can stop explosives or fire arrows. Combat is also very brutal and bloody. The battlefield becomes littered with bodies , gibs, blood, and it all uses phyics (one of the first games to do so IIRC, this was back in the mid-late 90's).
The game also when it came out was like the "half life" of RTS games, a huge mp fanbase and a ton of mods and coversions (from old west mods, to vietnam, new maps/units, etc). Sadly bungie.net is no more but that didn't stop them, you can play it online via mariusnet.com now, while it's not as huge as it once was there's still some die hard fans that play it online and you can generally find plenty of games online, if you wish to play it online that is. Also of note is that the single player campaign is fully playable co op.