I watched someone play through all 96 levels of Super Mario World a month or so ago... I never had an SNES, but the few times I tried playing the game at my friend's house, I never got very far.  It was cool to finally see the thing the whole way through, including all the secret levels.  I dig the idea that the map was incomplete and the player had to find all these hidden doors in order to get through the whole thing.

I also saw a playthrough of the Game Boy Super Mario Bros 2 Six Golden Coins, which I did have and which I did beat when I was a kid, but it was funny now seeing it, that it was the same design as SNES but on the Game Boy engine.  It was also real neat that every single level was a completely different design from every other level, with its' own villains, world elements, backgrounds, etc.  LOT of stuff for a Game Boy game.  There were even a couple of those same hidden levels.

On my Youtube app I've got a couple playlists of more games I want to watch: the Mass Effects, Arkhams, Grand Theft Auto V... all the big ones.  The trick is finding the time, these are LONG games!