1. I am grateful / obliged to you for …
2. I am greatly indebted to you for what you have done.
3. I am thankful / obliged to you for your unselfish assistance during …
4. I deeply appreciate your courtesy and we hope to have the opportunity of rewarding you rendered me.
5. I take this opportunity to express my deep appreciation of your kind help you rendered me.
6. It was kind and generous of you to do this for me, and I appreciate it more than I can say.
7. My appreciation to you for your generous help is beyond my words. I wish I could repay it some day.
8. Please accept my most cordial thanks for your timely help, which I will never forget.
9. Words fail me when I want to express my gratitude to you.
10. Again, I would like to express our warm thanks to you.