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There's a story here actually...
Back when I was a brand new stay-at-home parent I taught my toddler to say, "Thank you, you're the best mom in the world." as a kind of joke and because I was struggling with depression.
The joke stuck.
All my kids learned it. And they expanded on it.
My kids will throw out, "Thank you, you're the best teacher in the world!" or The Best Friend, or The Best Whatever without warning and with wild abandon.
The youngest is 12, the oldest is 22, they absolutely know this isn't the regular phrase. But it makes people light up. So they keep it.