Depends on what you're looking for.
ARMA II was just released. It's a first person shooter, but you do command a bunch of stuff. It's highly realistic.
Men of War is a realistic WW2 RTS. To put it into the RTS genre some realism was fudged (all weapon ranges are scaled down, for example). The single player campaign is pretty well done, but the voice acting is HORRIBLE (game was made in Russia and they didn't bother to get good voice actors)...but I don't see that as a huge issue.
Men of War is also a bit cheaper; $30 retail compared to $50 for ARMA II.