Super Mario World/Star World 3
Probably one of the most simple, but still tricky levels. Your main goal from the very beginning on should be either to run over to the right, exiting the Level without getting hit by any Spinies or to throw the Lakitu off of his cloud and take this all the way up to the top of the level, where the key will be hidden.
Note that in both cases, you can feed the yellow Yoshi until he reaches his full size with either the Spinies or the blue boxes (But make sure only to take the first row of those ;))
Of course flying with the Lakitu's cloud up to the top will let you proceed on the Star Road, while the other solution won't!
Don't worry about losing your Yoshi when you go for the key as you can easily come back for a yellow Yoshi.
Length-wise, it is by far the shortest level in the game.