I can also get behind the original Dawn of War including all of the expansions but Soulstorm; which had two interesting new armies but was pretty crappy. They should've just let us add those armies to the existing campaign. Dawn of War 2 is okay but I would wait until they expand it a couple of times. If you're just going to get one DoW, get Dark Crusade; it's engrossing, fun, and replayable.
Also, if you happen to have a PS3, I highly recommend Valkyria Chronicles. Parts of it are a little underworked but it's a very fun and original mixup of SRPG, RTS, and TPS. Also very re-playable.