There is no official line. The one most repeated outside the book, "They bark, Sancho, a sign that we are riding on," is not in it: the phrase does not appear in the Spanish text. Within the book, the most remembered lines are the opening, Sancho's refusal in front of the windmills, and the praise of liberty.
The opening is "In a village of La Mancha, whose name I have no wish to recall," in chapter I of Part One, in which the narrator presents a gentleman with a lance on its rack, an old buckler, a lean nag, and a swift greyhound, and refuses to name his village. At the end of the book, Cide Hamete also does not say where the place lies, so that the towns and villages of La Mancha may contend for the gentleman, as the seven cities of Greece contended for Homer.
The second is Sancho's, in chapter VIII: what can be seen there are not giants but windmills, and what look like arms in them are the sails, which, turned by the wind, make the millstone spin. The scene gave the language the expression "to tilt at windmills," and it sums up the book, which is the story of two descriptions of the same thing.
The third is the praise of liberty, in chapter LVIII of Part Two. Don Quixote tells Sancho that liberty is one of the most precious gifts heaven has given men, that for it, as for honor, one can and should risk one's life, and that captivity is the greatest evil. Whoever looks for a line about the gentleman himself finds another in chapter V: when his neighbor tells him that he is not Valdovinos or Abindarráez, but the honorable gentleman Quijana, he answers "I know who I am."