@resop2:
Not sure about a ranking, but for the hardest, I'll cheat a bit and put Karin's Usagi Tocome as the hardest song.
You may think happy bubbly pop songs would be easy, but I think Karin's Copink songs are arguably some of the most demanding idol songs out there, particularly Usagi Tocome.
Not only the range requirements, it also stressed on your musicality (especially if you want to sing the song just like Karin) and techniques down to the basic. From head voice to articulation to breath support and so on. Complete with a killer tempo, ridiculously fast monologue, and arrangement that would keep the singer on their toes throughout the song. You don't get much rest with this song.
I dare say, MoL is a walk in the park compared to this.