Current favourite words are "no" and "no need". At a tender age of 19 months, he already knows what he wants and would protest when things don't go according to his liking. We have been patiently explaining to him that he won't always get what he wants. Have told him umpteen times that he is too young to decide what he should/shouldn't do and thus, he has to listen to Mummy & Daddy. Yet, he still tries to assert his demands and would kick a fuss when we don't give in to him.
That aside, he can be a really good and obedient boy at times. I sometimes bring him to the toy store when we are at the mall and let him roam free in the store while I quickly browse through the toys. Compared to other kids, he rarely throws a tantrum when we have to leave and he obediently puts everything back. Even when I ask him if he wants a certain toy, he'd decline by saying "no" or "back back" (meaning to put back the toy). I guess he is still at the age where he won't demand for toys so I'd better enjoy this phase first before he starts telling me he wants this and that!