Company of Heroes is by far my favorite; according to Steam I have over 400 hours played in CoH: Opposing Forces. A lot of critics agree with me because CoH is the highest rated RTS of all time according to Wikipedia.
The main reason I like CoH so much is that the focus is on unit tactics and not which player can click the fastest. Resources do matter, but they rely on map control and not on the number of workers or money-producing structures you build. If you try to turtle and tech in CoH you will lose every game. Just blindly spamming doesn't really work either (aside from a few broken units *cough* Kangaroo *cough* which were eventually patched) when one well-placed machine gun can suppress several dozen infantry.
Also, I love the unpredictability of CoH compared to most RTS games. In Starcraft, if two opposing marines attack each other, the one who hit first is guaranteed to win the fight every time if neither unit is upgraded. In CoH that kind of predictability is thrown out the window because there are three types of cover, buildings to garrison, and a lucky artillery shot can sometimes wipe out an entire squad regardless of their health. The first combat units you get access to can be useful and deadly to the very last minutes of a game. Just ask any Axis player whose StuGs and Panzer VIs were immobilized by well-placed Vet 3 US Riflemen hiding in green cover throwing sticky bombs.
Finally, CoH has a great progression of scale. The small infantry battles of the early game are just as nerve-wracking as the late game when dozens of soldiers and tanks are duking it out amidst artillery bombardments and strafing aircraft.