An operatic space masterpiece
You have to keep in mind, back in the old days I had only dabbled in a bit of C&C red alert and Starcraft as far as RTSs go. They weren't anything special, I was far more involved in FPSs. So then, being a Trekkie... I go and buy Star Trek Armada. Ultimately a fun game, one of the better Star Trek games ever made, but pretty much a disappointment if you expected it to be more then Starcraft with starships. Then Homeworld 2 came out (I missed the boat on HW1), and what a revelation. Completely 3D environment, with no basebuilding, intelligent fleet formations and fighters all swarming around on their own. It was simply incredible. All from a completely new studio, out of the blue.
And the environments they created... were completely breathtaking. Every Relic game has had that unique Relic art direction that utilizes quality concept art to bring the game to life. It's completely unrealistic, but space never looked more beautiful then in HW2. FreeSpace games come close, but staring at some of the backgrounds as your battle-weary ships sail past... it's majestic, breathtaking, elegant, operatic, awe inspiring... like looking at a brilliant sunset in autumn. The grandeur of the cosmos was perfectly captured.
I dunno... it was the first, and still stands as the greatest 3D space RTS ever, as few of those as there are.