6. Resident Evil 5
Resident Evil 5 isn't just better with a friend; it's almost unplayable without. On your own, Sheva is a big, dumb bag of rocks Chris has to drag around with him while he goes about shooting half of Africa; with a friend she's your saviour - always watching your back and doing some smarter shooting on your behalf.
7. MGS: Peace Walker HD
Peace Walker HD replaced the original's local co-op with online co-op, and it isn't just an advantage; on some later bosses an extra player or two is almost essential. Peace Walker was built for four, with gadgets and weapons that only work with extra players in the room.
Like Diablo or Phantasy Star Online,Borderlands is about accumulating disgustingly powerful kit and adorning your character with weapons and armour so massive and conspicuous that everyone you meet reels in awe. Better yet, it's about scrounging swag from high-level players to cheese your way through early opposition.
9. Portal 2
Part videogame, part cruel psychological experiment, Portal 2's dedicated co-op campaign will very quickly test even the closest of friends' abilities to work together and solve complex tasks. Seeing asolution is only half the problem; working with another player is for the patient and forgiving.
10. HALO ODST
ODST is a selection of perfect Halo moments - a sniper level, a vehicle level, an all-out combat level - set between stealthy open-world sections that encourage creative co-op play. Its Firefight maps beat anything in Halo Reach, too.