The apples on the other side of the fence are the sweetest.
Literal
Fence's other-side's apples [topic-は] most delicious.
A proverb equivalent to 'the grass is always greener on the other side.' 塀の向こう側 is 'the other side of the fence.' 一番おいしい is 'the most delicious.' The implication: what you can't have always seems better. A culturally instructive sentence.