Resident Evil 5

This is probably the best co-op experience I've ever had in a game. No, really. A good co-op experience in a game is hard to find. So often, it just seems like you happen to be in the same area at the same time, doing independent things. RE5 has you and the person you play it with actually mattering, very much, to what the other is doing. My friend and I dialogued about strategy and planning, were yelling at the other for help, thanking the other for a save (Scripted or we just shot a guy about to hit the other) or a close shave, responding to nice shots or throws, asking for a heal, asking for ammo... etc.
Part of this is due to how Resident Evil is structured, though. You're not, as characters, great at dodging. Your partner shooting a guy behind you is really something that saves you, as you aren't going to dodge that. Your inventory is limited, so you can't both have all the guns (So, there becomes dialogue about who gets what). Ammo is scarce, so you have to work with your partner to make sure you both have ammo. Healing, as well, is done in a great way in this game. Any healing item used near your partner heals both of you for the same amount. Finally, though one of you dying results in you both losing, the game is littered with checkpoints, so you rarely lose much effort when you game over. Thus making it hard to get mad at your partner regardless of what happens.
Of course, this requires having a partner that isn't an ass. You have to compromise, share resources, etc. If you can get that, then you're golden.

The other important but non-gameplay aspect of RE5: Atmosphere/How scary it is. So, 4 and 5 are much less horror, more action. But, see, the only type of 'fear' that works on me in games is the fear of a big enemy about to kill the fuck out of me. And this game manages to achieve that pretty well. The game gives every boss the feeling of difficulty, all the big pitched battles feel rough, etc. It's nice, actually.

Really, the rest isn't terribly important. It's the least story intensive Resident Evil, the graphics are amazing, the music works pretty well, etc.

I do, however, have 2 more gameplay related things to talk about. QTEs and the final boss. And, actually, how they relate.
See, I don't like most QTEs. I like the QTEs in this game. They're not terribly common, add to the game instead of just fluffing out gameplay, and aren't abused. The ones in combat are all attack or dodge related. If you stun an enemy, hit X to QTE if you're in melee range and you do a special attack. Alternatively, any of a number of enemies will have you able to dodge attacks by various QTEs These are the faster, less telegraphed attacks. The cutscene QTEs are kind of annoying, but I can forgive them for not being overused ... and that it's kinda cool to see your characters die in the various cutscene ways. >_>
So, the final boss fight was horribly badly designed. At least in co-op. In Single Player, it's fairly straightforward, as you're only playing one side of the events. In Co-Op, though, you have to figure out, in the beginning, to run from the final boss, as shooting him then does no damage and you get no indication of this. And then, once you run, you sort of have to figure out exactly how the QTEs work and how to trigger the rest of the boss fight, again, with no real indication of how to do it. After about 6 tries of just getting through what felt more like shooting in the dark at how to get the arcane magic of this fight to go, we just checked GameFAQs and got along to the real part of the boss fight. It was pretty disappointing, all told.

Still, an overall great game for Co-op. Single player? I'm not so sure if I'd have loved it as much, to be honest. I might go through in single... Or I might just co-op it again with another friend. Maybe on hard, if the person can import a normal difficulty character. And I know I'll have a damned blast.