I'm in the middle of Red Dead Redemption and I have Mass Effect 2 in the works, but I also need something to just blow off steam.  Something short and punchy for those nights where I just don't have the energy for anything involved, or when I only have a 20 minutes or so to play.

As I said previously, I'm hugely addicted to Geometry Wars 2: Retro Evolved, but I'd like to try some other stuff too.

So what Xbox Live Arcade games do you like to play?  I'm looking for some quality suggestions.  I'll still download the demo to make sure it's right for me, but I don't want to miss something that doesn't sound quite as good as it actually is.  Some descriptions don't do a game justice.

Any sleeper gems that are hiding out there?  What's the best XBLA game that nobody's actually playing?