The fact that in Super Paper Mario, the music files are actually much faster than they play in game. This is done because they wanted the Speed Flower music to be sped up so the music file is sped up by default and slowed during gameplay. This...
I was born and still live in France to this day....
is this the part where I do some dumb joke like "baguette, fromage, croissant"? (even though croissants aren't even actually French but whatever...)
Probably what CM30 said. WL4 is the easiest and WL3 is the hardest due to it being potentially confusing. But all of those games are pretty easy regardless.
WL2 is also pretty easy I remember. You literally can't die and the game is linear.
Including Wario World and Wario: Master of Disguise?
Personally I'd say Wario Land 4 is probably the easiest one (at least on normal mode), while Wario Land 3 would be probably on the more difficult end.
But what do you think?