I had the exact same thought as Patrick regarding this, however with a little research I found that him nor I are correct. The most common used word in the U.S.A and even Worldwide is the word "OK". Studies show that the work "OK" is used more than any other.

Well, actually it'll probably be words like and, the, a, but, an, um (or uh), my, I, you, your, and down the line from there. But if you mean a term you fancy the most, I have a list as I change such words all the time.