Company of Heroes.
There is no question or hesitation when I say the following: Company of Heroes is the best RTS game on the market to date. It has so much strategy it makes other games look like economy simulators. It focuses almost entirely on the fun part of playing RTS games: the tactical strategy. There are very few games that do this quite as well as Company of Heroes does, the only other that comes close being Warhammer 40,000: Dawn of War. And, look at that, it's made by the same developer.
Relic has the right idea. Squad-based unit management, tactical strategy with cover and destructible environments, and so forth are what RTS games should be trying to emulate and use as a basis for success. By contrast, Starcraft II, while fun, is the same game that was released in 1997, only with better graphics. And, I'm sorry, but that just doesn't cut it anymore. It's strategic to a point, but after a bit, Starcraft just becomes an exercise in who can build the most units the fastest. And, really, that isn't what RTSes should be about. You should have to be tactical and strategic in the placement and use of your assets, not just churn out as many as possible and throw them into the grinder.
So yeah. Company of Heroes all the way.