This one was the breaking point of RTS games for me. The 3D epic battles where unit types, positioning, and tactics really mattered - including the sieges - blew me away. Add to that the grand campaign map which let you evolve your cities and family, whilst conquering the world with your nation (and reading some important quotes on loading screen) really brought this game to a peak. And the historical scenarios - whilst really difficult - were also very fun to play.
I still own a CD for the Gold Edition (which includes the Barbarian Invasion - also amazing - expansion), and hope to one day see this one and the whole series on GOG one day.