I really appreciate it.

我很感谢。

比较热情的说法：

You're one in a million.

你真是大好人。

You're the greatest.

你最棒了。

要归功于对方的时候：

Thanks to you (we made it on time.)

都要多谢你（我们才能准时完成）。

I couldn't have done it without you.

若是没有你，我不可能做到。

比较正式的说法：

I'm truly grateful for your help.

我非常感激你的帮助。

Your help was greatly appreciated.

你的帮助备受感激。

I'd like to express my gratitude.

我要表达我的谢忱。