World of Illusion. This was the first game fire dup on the Sega Megadrive 2 that my brother and I received for mas in 1992. After a hour epic trek around Manchester city centre trying to find said console (we looked everywhere, literally. No one had any left... except Currys IIRC), and many shop assistants trying to fob us off with a SNES (we were both Sega fans at the time... ah the innocence of youth) we finally got one bundled with Segas excellent Disney co-op platformer.
On Xmas morning at the frankly ludicrous time of 5am (as my mother had to go to work...damned NHS!) we set the system up, slipped the cart into the slot and pressed the power switch.
It is still the only time in almost 30 years of gaming that me and my brother have sat and played together without trying to kill each other. See, thats the magic of Disney!
We followed it with this:
Kept us busy playing in shifts the rest of the day..... Good times.