1) Total Annihilation (+ Core Contingency) - The only RTS you play just to enjoy the slugfest. First RTS where you had a full complement of land, air and sea units (instead of the half-assed 10-unit ground army, 3-ship navy and 2-craft airforce).
2) Warzone 2100 - Even if the time limits get on your nerves, it's still a good, different RTS. By the way, how do you get it to work on XP? The only solution I found on the net involved mucking something with Service Pack 1, which seemed a bit out of my league.
3) Dawn of War - Commanding infantry squads instead of single soldiers, and having some concept of cover and morale make a fairly traditional RTS seem fresh. And of course there's the close combat animation where an assault marine impales the foe with his chainsword.
Medieval (particularly Viking Invasion) and Rome: Total War are both lovely, but I don't really consider them RTS games, they're too much of "real" strategy games to get lumped in the RTS genre

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