Easiest:
A Song for XX
Beautiful Fighters
GAME
Replace
Trauma
my name's WOMAN
Hardest:
All the rest
XD
Just kidding:
M (I can't sing the 'MARIA' part to save my life)
INSPIRE (My voice is too damn deep to sing low in the verse then high in the chorus X_X)
evolution (Too fast)
Ladies Night (Verse is too fast)
fairyland (Just can't sing it
)