The satisfaction of completing the MasterStages in the original Super Monkey Ball for the first time. Even reaching them is a feat in of itself!
Reaching (and beating) the Master Stages of any of the games can aslo count as this, doubly so if they manage to beat them without loosing a continue or even life.
In Super Monkey Ball 2, Dr. Bad-Boon has used a shrink ray to shrink Aiai, Meemee, Baby, and Gongon. Bad-Boon gives Meemee an ultimatum: either marry him or watch her friends be cooked. Meemee just snaps and flies up Bad-Boon's nostril, making him sneeze, which blows away the other monkeys, giving them a chance to escape.