For me, though I haven't played it in ages, Star Wars: Force Commander. This game got trashed by basically everyone but I found it very enjoyable. The pathfinding was pretty atrocious, but if you microed like a freak you could work around it. Personally, the SP campaign is pretty much the best I've ever seen for an RTS. The multiplayer and skirmish was lacking, but the SP campaign was long and very high quality IMO with interesting scenarios beyond just "destroy everything". I also enjoyed the way you could carry over experienced units across from one mission to the next and thought the CQB combat within buildings was really cool. Also just really fun units in general. One of the few RTSes from the past I'd play through again.