Couldn't be better/worse

How's it going? (It)couldn't be better.　最高だよ。

How are you doing? (I)couldn't be better.

How are things going? (Things)couldn't be better.

How is your married life? (I)couldn't be happier.

How are you feeling today? Couldn't be worse. 最悪だよ。

I think Japanese railways are the most punctual in the world.　I couldn't agree more. まったく同感です。

punctual パﾝﾁｭｱﾙ　adj)時間を厳守する、規則正しい、点のような

It couldn't be better.　あれ以上のものはない。

It couldn't have been better. (過去の出来事）あれ以上のものはありえなかった。