be most(/extremely) grateful ❍ 您多给美言几句，我～！ (老舍《茶馆》25) Put in a good word for me and I’ll be most grateful!/弟即修字数行，烦尊纪带去，便感激不尽了！ (《红楼梦》1465) I shall write a few lines to her,and if I may trouble you to pass on my letter I shall be extremely grateful!
感恩不尽ɡǎn ēn bù jìn
形容感激不尽他人的恩德。be everlastingly grateful, be most grateful