For sure, Civ 4 & 5 weren't the great games they are now until the 2nd & final expansion for each—and of course, the few bug fix patches thereafter to clean things up.4X genre doesn't seem to be able to do without them
I have to go old school.
As much as i like Vanilla StarCraft, I'd have to add brood war to this list. The huge campaign, with the addition of those key units made SC turn into the world wide super hit that changed gaming for ever. It's kinda hard not to put this one at the top.
But you still know what parts of it are and aren't part of the DLC, right?