I recommend Company of heroes series. There is free beta on Company of heroes online going on. Download it and see if you like it. There is link to their site: http://www.companyofheroes.com/ you can download it from there.
World in conflict is probably still the best available. Don't follow the RTS scene too much I have to admit - but compared to dozens of other games, the game design is just outstanding in this one. Quite a bit of depth, high stress on teamwork, and there is almost never more than 10 seconds when no interesting action takes place. Quite fast paced - a game is 20 minutes tops.
Set in late 1980's cold war setting, combined arms approach. You can command forces as the armor, support, infantry or air arm of the army.
IF you want a high degree of realism I would suggest Stronghold 2, it is available from steam, and it also gives you a challenge to play, as you will have to be managing everything in town such as resource, waste management and thievery, etc.
I is really a fun game to play, and you can play it only ( It is based on the medieval times)