One-word quotes are tough. You could argue that the only single common word quote that qualifies as a real quote is "plastics," unless we have a made-up word or unique name ("Rosebud").
This is an iconic use of the word, which I am sure you will agree when you think of the answer. In fact, I will go so far as to say, it is the most famous one word suirrel quote in all of film history! Think on it a little longer, and if you don't get it, I will give you a big hint.
"Now, there comes a point where a reasonable man will swallow his pride and admit that he's made a terrible mistake. The truth is, I was never a reasonable man"