When answering a letter, you should always be polite. The following sentences may be useful for you to expresss thanks to those who wrote to you.

Thank you very much (very, very much) (ever so much) (most sincerely) (indeed) (from the bottom of my heart).
很(非常)(非常非常)(最真诚地)(确实)(衷心)感谢您。

Many thanks for your kind and warm letter.
感谢您友好而热情的来信。

Thanks a million (ever so much).
万分(非常)感谢。

Please accept ( I wish to express )my sincere (grateful) (profound) appreciation for ...
请接受(致以)真挚的(衷心的)(深切的)感谢……

I sincerely (deeply) (warmly) appreciate ...
我真挚的(深深的)(热情的)感谢……

I am very sincerely (most) (truly) grateful to you for ...
为了……，我非常真挚的(深深的)(真诚的)感谢您。

There is nothing more important (satisfying) (gratifying) to me than to receive one of your letters.
再也没有比收到您的来信更使我觉得重要(快慰)(感激)了。

Your letters are so much fun (comfort) (entertainment) (company).
您的来信充满了乐趣(给了很大安慰)(带来了欢乐)(使我不感寂寞)。

Your most courteous (considerate)(delightful)letter...
您那彬彬有礼(体贴入微)(令人欣慰)的来信……

I can not tell you how much your letter delighted(relieved)(amused)(enchanted)me.
我无法告诉您，您的来信使我多么高兴(宽慰)(觉得有趣)(陶醉)。