I think there's a curve effect with Paradox games.
Here's a crappy graph to show what I mean.
The games pretty much always get more refined over time but the popularity is bound by a cross section of newness and refinement. At first all the games are really new and explode but aren't refined. Then they enter a lull because they're still not refined but also not fresh off the presses. It comes back though after the 4 or 5th dlc. By the last few dlc, as we see with ck2, the game is fully refined but is getting old and people are looking towards the next new thing. I think all PDX GSG games will follow this curve as long as pdox's dlc policy is in place (not that it's a bad thing).
My prediction is that in 3 years eu4 will take ck2's place, hoi4 will take eu4's place (and stellaris will be just slightly behind) and rome 2 or vicky 3 or whatever is next will take up hoi4 and stellaris' spot. ck2 will enter the nether realm of 'It's a classic but I can't wait for the next one' that vicky 2 currently resides in.
So in the end imo the best games go vicky2 > ck2 >eu4 >hoi4> stellaris and they'll all move up one bump in a year or two.