The word/phrase you use constantly.
I like to think I don't use any words to the point that their use can be described as "constant." Some words, though, do seem to pop up when I talk or write. The main word is probably "just." I catch that one a good bit in my writing, so I suppose it's in my speech, too. However, the one I know I use with some regularity is "really." "Really" has to be asked with the all the sarcasm and irritation of an adult and is most useful when confronted by the unbelievable action of another. For example, I've just finished mopping the floor and someone walks across it in muddy boots. My response is, "Really?" With practice you can pack a lot of emotion and meaning into that one word with facial expression and tone and it has the added bonus of being irritating to the person who annoyed you to begin with.