You should try Attack On Titan. It's about the last city of humanity and its struggle to stay alive against the Titans, a race of giants that look like deformed, genderless, humans. They are always smiling, completely mindless, don't need to eat but chase down and devour all humans in sight for what seems to be the pleasure of it. Since they don't have digestive organs, once they're full they throw the people up and then go looking for more. A great feature about it is how interesting and different the secondary characters are and that the women are capable and aren't crouched in a corner crying about how useless they are which is an extremely annoying anime status quo.
A warning though, it's extremely gory. Since only a few humans have the skill and power to kill a Titan, most battles with them end with a LOT of less gifted soldiers being maimed and killed.
