Part of the answer is up to you. If you really like military stuff, than you should consider getting Zombies!!! 2.
From adding to the game, I like Zombies!!! 3.5: Not Dead Yet
which is the all card expansion. It lets you alter the mix of cards in the game by taking some out and replacing them with something you like better in the expansion.
i also like Zombies!!! 5: School's Out Forever
which introduces the guts mechanic which allows you to modify your or your opponents hand size from one to five cards through new cards and the impact of die rolls.Zombies!!! 6: Six Feet Under
is also cool. It adds sewers and subways to the normal map allowing characters to move much more quickly between spots on the map but with some chance of dying if you're underground too long.