As I'm sure will be suggested a few times, but Resident Evil 5 & 6.
Resident Evil 5 is a bit more all-around solid, not really a horror game but quite a fun arcade-shooter with a pretty competent campaign. Lasts a good 8-16 hours to beat the campaign the first time, and there's a ton of unlockables (costumes, weapons, a new difficulty), on-top of Mercenaries Mode, which is easy to sink onto. Also get the Gold Edition, as it includes the DLC, including the fantastic bonus two story chapters (Lost in Nightmares and Desperate Escape, both of these add an additional hour or two and are probably the best chapters in the game as they responded to a lot of criticisms to RE5, as well as they come with more costumes, and a new Mercenaries Reunion Mode, which is essentially a harder version of Mercs but with more broken characters, but characters not in regular Mercs mode). It gets a lot of slack, but it honestly is one of the best co-op games around, and is both local and online.
Resident Evil 6 is a bit more uneven, it in my opinion has better highs and lower lows than RE5. It essentially has so many ideas stuffed into it, so literally almost every chapter in the game is completely different. But part of this uncertainty of what's to come next is fun in co-op, and you'll do everything from fight zombies in a neighborhood, to performing stealth, to piloting planes, to fighting giant sharks on waterslides, and a lot more. The combat is really good too, though it takes a bit to adjust too. I might suggest playing a bit of Mercs first to adjust before hoping into the campaign. Speaking of which, Mercs is awesome and worth experiencing. Though some of the stages on PS3 are DLC. I prefer it in some ways to RE5, but it definitely is a less steady, more uneven experience, that sort of has a love it/hate it status right now with gamers. However, a lot of bang for your buck, it has four completely different campaigns. and beating the story in the game should take 15-25 hours. On-top of this, there is an addicting Mercs mode, and a few small fun but throw-away modes, like a mode where you play as monsters and invade other peoples campaigns. Local and online again.