If you haven't played the Shadowgun: DeadZone beta yet, go do that. Well, if you have a tablet (or Galaxy Note), that is. It's pretty hard to play on a phone. But this is bar-none my current favorite game for Android, and I'm fragging away at it for probably an hour or so every day lately.
The game has been in open beta for around a month now, and after several patches, is apparently just about ready for prime time.