By far the game that gave me the most unexpected fun was Earth Defense Force 2017 on the 360. It's a very old-school type of shooter, but incredibly addicting.
There are 53 missions, 5 difficulty levels, and over 170 weapons to collect and use. Once you complete the game on the hardest difficulty, you unlock the most awesome weapon I've ever used in a videogame - the Genocide Gun. It's like a shoulder-mounted nuclear missile.
There are vehicles to use but they're not all that useful, and there's also splitscreen co-op, too. The only negative is that the game doesn't have online play, which would absolutely rule.
You can probably buy it brand new for $10. I seriously cannot recommend the game highly enough, especially if you're a fan of old-school arcade action.
