I like league of legends, It's really awesome if you can somehow assemble a full five people within a reasonable skill band.
But that is impossible. You can duo que for 5v5's but it is annoying compared to solo que. Still works though.
It's free.
Genuine teamwork eh, Guild wars has a decent amount of teamwork, like, stop kiting and run back to me so I can healz you, or focus carefully to interrupt the critical spell of one guy. Duoque Guildwars is FUN. Both vs normal enemies, and in 4v4 Which has a learning curve up to a point, but then is somewhat reasonable. I have not played guild wars 2 though, and you WILL want to play Guildwars 2 (so you aren't stuck with a team of trolls).
Advantages: Far as I know, guild wars 2 is still the only MMO with a flat fee.
You'll want to buy expansion packs anyway though, *shrug.
Halo 3 had a decent co-op but true full on teamwork? Not in campaign mode. I hear borderlands is really awesome, but I haven't actually played it.
New super mario bros is absolutely rediculous co-op, but it's not about cooperation thaaat much, mostly just not stepping on your friends head. Or deliberately stepping on your friends head.
I watched the whole series of naka teleeli playing Lost Vikings and it looks Incredible, But kinda too hard for more than one person. Damn candy levels. Definately something hardcore. To be fair, they were playing online through an emulator.
Trine looks interesting, I don't know much about it.
BTW, is there a game too difficult, long, or boring for you to play?