expression
really good; irresistible
Used to express that something is so good or appealing that one cannot resist it. Often used in casual, enthusiastic speech.
このアイスクリームはこたえられないおいしさだ。
This ice cream is irresistibly delicious.
夏のビールはこたえられないね。
A beer in summer is just too good, isn't it?
Both express something irresistible, but たまらない is more common and can be used for negative feelings (e.g., 寂しくてたまらない), while こたえられない is almost always positive and emphasizes enjoyment.
From the negative potential form of 堪える (to bear, to endure), literally 'cannot bear/endure', extended to mean 'so good one cannot resist'.