I'm much obliged to you for helping us.

承蒙相助，本人不胜感激。

We are very much obliged to you for doing us such a service, and mother would like to see you.

你帮了我们这么大的忙，我们非常感激，我婆婆想见见你。

I'll be very much obliged.

无任感荷。

I am much obliged to you.

我万分感谢你。

I'm much obliged to you.

你太好了，多谢你了。