My favourite RTS of all time is Company of Heroes.
I just love how a single squad in the right place with the right gear can do a serious amount of damage.
The game isn't about how many zerglings you've got vs the other guys zealots, its about positioning and knowing strengths and weaknesses and that sorta thing, even moreso than other strategy games.
My 2nd favourite RTS is Dawn of War, I love how early on there will be small fights over strategic locations, then all sides get their main infantry into the game, then everyone upgrades them, then heavier units get sent out and it goes from scouting and tactics to a huge bloodbath within one match.