I've played 2, 3, and 4 so far... 4 has been my favorite. Some of the new systems that were introduced make the gameplay that much more enjoyable and introduce some new-ish themes to the game. They've also hammered down on their older systems which make some functions of the gameplay less tedious.
I need to get back into it, very fun game to get lost in for several hours.
I saw it when it was first released (Yay WP/W8 got something first), but it looks too much like another Clash of Clans for me to get too excited. I lost interest in CoC, this doesn't look to hold it either.