As if I need to put any thought into this.
Toejam and Earl 1. The first game I ever saw two players cooperatively helping together to share presents/items and one of the first cooperative games I've ever played. Instead of it being a competitive game, you actually helped eachother.
It's rare, I know, but there isn't a large supply of games where they're 1) cooperative and 2) able to have a single player on a certain level while the second player, on an entirely different level, roams around.
In fact I've never come across another game quite like this. It's relatively long (25 levels) and it can last you a good night to play through it by yourself. The random map generator is also great, so you technically aren't playing the same game over and over.
There's also Ecco, the first title, but I wouldn't call it perfect. Near perfect maybe. Same goes for the likes of Chrono Trigger, Super Metroid, Rayman, Lemmings, and so on. But they all have their flaws.
Then again I'm sure Toejam and Earl isn't without it's flaws either. It's rather slow and that irrirates some people. Other people just can't stand the amount of roaming around and searching the game has. Plus, you can kill yourself by opening an unsuspecting present which flat out electrocutes you to death. But uh, yeah I don't really see these as huge issues.
Jam it back in, in the dark.