Easily Company of Heroes. It is not even close as it is on a level of its own. Also the long anticipated sequel for it will be out early in 2013 so you picked the perfect time to get interested.
Yeah I like Company of Heroes for the reason that it is the most realistic of any of the RTS games. I have the primal game plus one of the expansion packs as well. The skirmish games are awesome and I love the unique abilities each country has at their disposal.