The Total war series.
Both shogun and Medieval are great games but Rome Total war will rule the RTS world.
It has the empire building of civilization and real-time battle were you actually have to use tactics to win unlike AOE,WC and SC where you just build a big army(even tho you can only controll 50-200 men) and attack.
Flanking, taking advantage of the hills and trees are useless in those games but in Total war series you can defeat any army even tho it is 3 times larger then yours if you use these elements.