The confusion of the possessive "its" (no apostrophe) with the contractive "it's" (with apostrophe) is an unequivocal signal of illiteracy and sets off a simple Pavlovian "kill" response in the average stickler. The rule is: the word "it's" (with apostrophe) stands for "it is" or "it has". If the word does not stand for "it is" or "it has" then what you require is "its". This is extremely easy to grasp. Getting your itses mixed up is the greatest solecism in the world of punctuation. No matter that you have a PhD and have read all of Henry James twice. If you still persist in writing, "Good food at it's best", you deserve to be struck by lightning, hacked up on the spot and buried in an unmarked grave.