Probably the sonic rainboom in "Sonic Rainboom." It's as awesome as it is colorful.
To be honest, I think that every second of every episode is visually impressive. Beautiful animation, colorful and vibrant character design, cool backgrounds, etc.
My favorite have to be Burger King fries when they're cold. I just love them and could eat them all day long. As for the worst, probably the fries at Arby's near the Duluth mall. For the record, Arby's burgers are okay, but the fries are so salty it's just unbelievable and makes my eyes water. What were they thinking???