Pretty much as the title says, what's your favorite co-op game, either online or local?

For me I'd have to say anything strategy involved like Ghost Recon: Future Solider, I love horde mode in that game with a local co-op and planning the perfect defence, preparing long distance support as well as close quarters. Always intense. I also enjoy more tactical games like Frozen Synapse.

What about you?

EDIT: I forgot Demon's Souls/ Dark Souls!

EDIT 2: I just remembered, and I'll probably get hated for this but SimCity (2013) is fun with a few friends once you get around the bugs of sims not working in other cities and such.

Mario. *ducks head*

I love being able to grab my girlfriend's character and throwing it into an enemy or down a hole. We're very competitive.

I gotta go with Portal 2. My friend and I have a blast anytime we play this online.

I used to be into Payday and L4D2, but my friends don't play them, and I get tired of people screaming at me all the time. So I don't play those anymore.

Back in the day, it was always fun playing Contra on NES with my older brothers and sister.

L4D2, counter strike 1.6, Age of empire 2 playing against the pc with a friend is very fun too :D

I enjoyed both Borderlands. The checkpoint thing is very annoying since my friend and i sometimes don't have the time to finish a mission and we get sent way, waaay back to the last checkpoint if we don't reach the next one. Other than that, it's quite entertaining in co-op.

I don't know what my favorite co-op game of all time is.. but my favorite recent co-op game is Resident Evil 6. I loved it, and think the co-op is a lot better than the reviews give it credit for.

2nd favorite would probably be Hard-Corps Uprising. Not many games of this type have online co-op, and Hard Corps Uprising does it extremely well.

Hard to choose, but it is probably Monster Hunter to me. So incredibly fun to play with a bunch of people.

Sadly my friends don't like it so I rarely play it with others.

Sex.

Local play of course, online just isn't as fun.

Have to agree with Cormoran, but can't judge about online version since I tried only local play.

Some blasting Co-Op fun from time before discovering above-mentioned:
Streets of Rage 2&3 (SMD2/Genesis)
Twisted Metal 2 (PS1)
Fighting Force (PS1)

Has to be Borderlands 2 as you can just drop in and out when you want, its fun and frantic, great when its 2 different characters also.

Love playing Streets of Rage co op with my son on the Megadrive also

I'm more into beat-them-all like TMNT IV on the SNES and Streets of Rage 2 on Sega Genesis. These 2 games are the best beat-them-all of all time for me and I never get tired of playing them.

And with a friend it's downright awesome.

Also there's a little plateformer I never get tired of playing despite being short and ultra easy : World of Illusion starring Mickey Mouse & Donald Duck.

This game is just charming and the co-op is clever and funny. Truly one game you should try with a friend for an unforgettable experience

Hm... tough question. Next one!

Nah, a game I really enjoyed playing coop over LAN was Operation Flashpoint, haven't tried multiplayer in any of the later ARMA games. We also used to play the Invasion mode of Unreal Tournament 2004 a lot, particularly when I found the Vehicle Invasion mod which allows to play any map in invasion mode (at least any official one). We just had a blast playing that one, sometimes combined with instagib mode. Oddly enough even the Mothership map worked perfectly and provided some of the most intense battles ever.

Then there was also Vietcong (which I keep mentioning a lot, lately) and Hidden & Dangerous 2: Sabre Squadron (which allows to play the addon campaign in coop mode). Particularly H&D2 was a nice surprise. It was only the two of us but we ended up both trying the most desperate, idiotic and *fun* things as well executing the occasional brilliant tactical plan.

For some time I enjoyed playing L4D with random people, more recently Dead Island and Riptide which - call me crazy - we enjoyed more than Borderlands.

WoW

I really enjoyed Dungeon Siege I & II co-op. The story is completely forgettable, so you can just concentrate on the action and work in tandem with your partner to maximise damage with different classes.

Rock and Roll Racing (not really co-op if you don't want it to be) gets an honourable mention. It's a SNES game, so it's probably not the best recommendation.

Hmm... i'd have to say NwN 1 is my all-time favourite just because of the sheer amount of time i've put into that game (played it for 8-9 years). Easily one of the best multiplayer modes for an rpg ever, imho.