In terms of 2D this is super tough.
I just cannot choose between Mario 3,Yoshi's island and Mario world.
------------------------------------------------------------------------
They are very addictive.
Mario 3 is the easiest by far.
Although there are many hidden whistles and 7 coin ships.
---------------------------------------------------------------------------------------------
Mario world for me has the most wonderful music each type of music suits each round to perfection.
There are many secret exits from various rounds.
You also have Star world and Special world.
Yoshi's island has great music and you can change into many different things.
-----------------------------------------------------------------------------
A helicopter being one of them.
Its very challenging trying to get 1 hundred per cent in each round.
To do this you must get 30 red coins,four flowers and 30 stars.
All these are masterpieces made in Nntendo's golden age.
Here is some Mario world music.
http://youtu.be/FrC2e4R5Uq4