Well, I don't think you should mean Shortest as in leat amount of level. I think it should be least game time.
Sure SMB can be beaten by a novice now a days in mnaybe 10 minutes if you take the wartp pipes and run all the time. But if you're playing for the first time, it could take a while longer.
My opinion of the shortest Mario game would have to be a toss up between 1 and 2 (USA - i.e. the Toki Toki one)
They both can still be beaten relatively shortly even if you go through every level. Whereas SMB 3, SMW, etc after that all had more levels and more secrets and sometimes a map allowing you to pick your level (And sometimes replay it for bonuses).
So, I'd say SMB 1 and SMB 2.
I will not rest until Mario and Luigi get a REAL 3d Mario Bros. Game.
