What is the "superlative" way of expressing "thank you"

Solution 1:

You could use "Many thanks". This would sound weird in person, though.

Also, emphasizing with another sentence would work: "Thank you. I genuinely appreciate it."

Solution 2:

Being that the "superlative" form of some idea is one in which the intensity or strength is at its greatest (an upper bound in some sense), I believe logically we're left with no choice but:

I cannot thank you enough.

Solution 3:

In order to make Thank you a more serious phrase, you can say:

Thank you very much

This adds an extra layer of expression, which most (if not all) people will interpret as implying a higher level of thanks. You can use the same phrase in face-to-face conversations, and you can add further gratitude by using the same tone of voice as you might when saying a highly grateful "Thank you".

This is the highest level of thanks I have seen expressed, but the right tone of voice and body language can make this carry a lot of weight. I don't think people will think you are exaggerating--the phrase that can throw people off is just "thanks", which may sound curt.