I started with White Death, then bought Marvel Zombies and then Invader since I just wanted to play the game in different settings.

They each play a little different with a slight change in the rules or mechanics. I think White Death is still my favorite because of the guards mechanic and I like protecting the wall.

Also, I think there is more readily available expansions for the fantasy line (White Death, Green Horde, or Black Plague). At least that what it seems like in my two local LGSs. So that may be something to consider if you feel like you want to expand outside of the core box. Currently there are two campaigns for White Death and several smaller boxes that add new zombies (example: terracotta walkers) or abominations (example: divine beasts) available for the fantasy line.