No matter how good someone or something is, there is always someone better; used to express that excellence is relative and to warn against complacency.
彼は野球が上手だが、上には上があるから慢心してはいけない。
He's good at baseball, but there's always someone better, so he shouldn't get complacent.
新しいレストランは評判がいいが、上には上があると聞いて、店主はさらに改良を続けた。
The new restaurant had a good reputation, but hearing that there's always someone better, the owner continued to improve.
大学のランキングで上位でも、上には上があるから安心はできない。
Even if a university ranks highly, there's always someone above it, so you can't be complacent.