Excellent question. I’d nominate a few words for this distinction, but there is one that is just so blatantly overused, I’m going to designate it as the one word too many people use.
First, however, let’s look at the “nominee” words:
UM – In speaking (particularly in public or in front of a crowd of any kind), I’ve found I love using the word “um” before I launch into a sentence…way too much and way too easy! Additionally, you could argue that it is not even a word.
SORRY – Not so much a word over-used verbally in conversation as much as it is used to be a band-aid for every type of offense. Bottom line to me is to say it only when you truly mean it.
WHATEVER – If your intention is to end a conflict, this is probably not the word to do it with. This is the grammatical equivalent of throwing gas on the proverbial fire.
THE SCIENCE IS SETTLED – Not a single word, but I think it’s a phrase that is used way too much to shut down a disagreement about an issue, usually a political one. The very nature of science lends itself to never being settled, as new information may become available that would change the hypothesis.
FINE – Nothing says things are not okay quite like the word “fine”. In most situations, fine literally means the precise opposite of its Webster definition.
So, what would be the word I feel is most over-used by people in everyday conversation?
LIKE
Like, what is your like, favorite word in like – the English language? Like, I’d have never guessed that! It’s like, so cool to know that bro!
Need I say more?
MOTEVENTURE 
I remember when “like” first came about in the 80s. The movie Valley Girl painted a slightly exaggerated but also somewhat accurate picture of a small pocket of LA. Soon after that, it exploded
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Well done
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