If you are praising someone then you would say; "I appreciate you.", or to give high praise, "I really appreciate you." "must" defines something that you have to do, without a choice. Appreciation must come from within and cannot be commanded.

No, it's better to say, I really appreciate all your help/everything that you have done

"That's much appreciated." That is the shortest phrase I can think of.

You can say: 1. "I appreciate what you've done for me". 2. "I'm so grateful to you ."