We're not big fans of the tower defense genre, but there's just something about Plants vs. Zombies. Defend your garden with plant warfare against seemingly endless hordes of zombies by obtaining points, upgrading your plants, and creating the ultimate plant fortress. We seriously recommend Plants vs Zombies 1 over 2, as 1 is not nearly as bogged down by micro-transactions, typical of the "freemium" model.
Google Play link
