Best XBLA game

[Zhukov]

Let's say I only have enough credits to download one Xbox Live Arcade game (because I do), what would you guys recommend?

I'm interested in a really good co-op shooter/beat em up, or just any great game in general.

What are your favourites?

I've already got Castle Crashers and Alien Hominid, and I could go something similar to that. I was thinking Either Metal Slug 3, Turtles in Time or Warhammer 40K: Kill Team. Ikaruga is also appealing.

Suggestions please.