Easiest has to be Go With The Flow Combo. Mastered and played Master Performance for 247k points the first day I got the game.
Hardest is prolly Hanger 18 or Painkiller. Can't put either one over the other in difficulty as they're both beyond my ability atm in certain sections. Haven't unlocked SSB yet, and now hat i'm getting freezes again i may never. <_<