Alas "guaranteed" is a harder thing to find if it is a random stranger you want to get co-op sorted with -- I do not know if you have ever tried to find a good few randoms to do some kind of tactical game, now think what might happen if you want that for 10 hours for co-op. To that end if you can find a friend or family member then I have found that the best path. To make matters worse you will find some of the better co-op games are levelled and that makes things even harder if you are trying to find someone you own level or near enough -- Borderlands (1,2 and pre sequel), The Lord of the Rings: War in the North, Dark Souls 2, Sacred 2 (don't do 3), Diablo 3 and technically even Resident Evil 5 (6 is not the best game, though it can do slightly better for co-op) can be a pain. Other than Diablo 3 I have had a great time playing all of those in co-op in various forms, I just did it with friends and split screen.
Such things are usually also what I spend most of my time doing co-op on so while I like the Earth Defense Force franchise, which should dodge most of that, I have no idea how many people are playing it online these days. Equally the Operation Flashpoint games are fun if you like that sort of thing but I never really liked the console versions and trying to get people to play properly online is no fun at all.