Camp Keep a Good Mac Down
When Eduardo keeps going on about some forest monster and Mac tries to explain to him that there's no such thing. However the more Eduardo goes on, the more Mac starts to believe in it and finally panics when Ed says the monster loves to eat 8-year old boys the most

I LOVE that scene
Bus The Two of Us
Right before Bloo scrashes into the schoolyard Mac is playing with some toy cars with some other kid going "WROOM WROOM! SCREECH!!" until the other kid interrupts him. I just love the fact that he's in a sand box
Mac Daddy
Bloo freaks out and Mac tries to fix things, explaining to him he didn't know how it happened and that it was a mistake and all that

you can REALLY feel the agony in that scene! from the both of them... but especially Mac for feeling like such cheater XD