I know it's not the co-op you're thinking of but Buzz Quiz TV is great fun to play when you have friends around. There are question packs to suit everyone's knowledge base.
Valkyria definitely.
I really don't think you can go past LBP for 2-4 player local co-op. And fun online co-op too. It's the most fun you can have on a console without killing/punching things. And more fun that a lot of killing/punching things games too.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix








