I really appreciate it.
我很感谢。

You're one in a million.
你真是个大好人。

You're the greatest.
你最棒了。

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。

I'm truly grateful for your help.
我非常感激你的帮助。

Your help is greatly appreciated.
非常感激您的帮助。 (很正式!)

I'd like to express my gratitude.
我要表达我诚挚的谢意。 (相当正式!)