Is it correct "I must appreciate you"
Can we say like this to someone in order to praise him or her?

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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.

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

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"That's much appreciated."

That is the shortest phrase I can think of.

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You can say:  1. "I appreciate what you've done for me".

2. "I'm so grateful to you ."

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