An expression meaning 'extremely ...' or 'as ... as one could imagine', used to emphasize a very high degree of a feeling or quality.
彼の失敗には驚きといったらない。
I was extremely surprised by his mistake.
そのケーキは美味しいといったらない。誰にも教えたくない味だ。
That cake is indescribably delicious. It's a flavor I don't want to share with anyone.
台風の被害は大きく、困難といったらない状況だ。
The typhoon damage was severe; it's an extremely difficult situation.