Tactical Soldier has been garnering some acclaim on the App Store lately, and it's well deserved. It's a turn-based strategy game that's reminiscent of the old X-com series that combines a horror story with turn-based tactical gameplay, sending you to guide your team through a full campaign exploring a military base overrun by the walking dead. You'll use various weapons to take out zombies while trying to keep your forces in formation and lined up for attack.
The game's fun, though it suffers from some of the usual tactical strategy problems, including a super sluggish pace at times. The interface is clean and easy to use, so the only headaches will come from having to wait around as some of the levels drag on. Even then, this is easy to recommend for anyone interested in tactical strategy experiences, especially since there are some fun upgrade systems and power-up items.
Tactical Soldier is only for the most recent iOS devices because it does draw quite a bit of CPU power with all of those zombies running around. Game Center integration is included for leaderboards, but there is no multiplayer yet; the developers have promised more content is coming in the future. It's available on the App Store right now for US$4.99.