How old is old, anyway? Because recommendations vary depending on how close to the present we can get.
Regardless, though...Myth, Myth, and more Myth. The first two games are seriously real contenders for Top 10 Best RTS Ever. They're really more tactical than strategic games, but I seriously can't hype them up enough. Before Starcraft was even a thing, and IIRC before the supremacy of AoE, Myth came out with an incredible story, very good music, and some super hard but amazingly rewarding and memorable gameplay. Anybody who played it and hears 'Casualty!' is going to get nostalgia-bombed really hard, and for good reason. If you ever wondered why people whined that Bungie had gone to **** when they first started making Halo, these games are why - nothing they've come up with since then has had such a long-lasting appeal.
--
Twostuck, Homestuck RPG: http://www.gamefaqs.com/boards/8-gamefaqs-contests/62168402