Company of Heroes hands down.
World War 2 might be overdone in terms of the gaming genre, especially in the FPS arena but Company of Heroes is much different. In my opinion, the best RTS around successfully introducing several extremely good and realistic things into the RTS genre (Supply Lines, Cover, directional armour, chance to disable/damage vehicles ie. destroy main gun or damage engine).
The Campaign is superb. Computer generated movies with compelling narrative are punctuated by animation from the ingame engine to create atmosphere and the tactical overlay of mission objectives immerses you into the game, making you feel more like a Commander receiving orders. The multiplayer community is strong and well developed so whether its singleplayer or multiplayer, you are well catered for.