Super Mario Galaxy 2's The Perfect Run has to clearly be the hardest, simply because of the fact that taking damage even once equals an instant life loss.
The Endless World of Yoshis was a nightmare when I was doing my hundred percent run of GBA Yoshi's Island. The New Mario 2 Impossible (which would be more aptly named the 'improbable' pack) pack is alone can match it.
Neither of those are in the poll. But even among the choices in the poll, one thing they all have in common is none of them are required to beat the main story. In conclusion, anyone who says Mario games are easy are shutting them off after the end credits roll.