Total Annihilation made a break from the conventions of the RTS genre by providing infinite resources, practically infinite unit variety (with the expansions), and a seemingly endless number of strategies in a game that could become a real marathon meatgrinder. The original is still superior in many ways to its spiritual sequels, Supreme Commander 1 and 2. But what it and its unofficial follow-ons all share is a visceral battlefield full of rocking artillery, explosions, machine-gun fire, and mayhem.
Unfortunately, outside of the campaigns, the AI had a nasty tendency to send its commander out on a suicide run to guarantee a stalemate. You can get around that by turning off the option to end the game with commander destruction, but then you are REALLY getting into a slugfest if it doesn't end with an early rush.
But those are minor issues. This is simply one of the classics - easily up there with the original Starcraft and Age of Empires II in my mind.