The Show Best Word booster unlocks at Level 46, and it will BLOW YOUR MIND! It will find the highest-scoring word among the face-up letters on your board and the draw pile, and assemble that word in your letter rack. (You will still have to tap the Submit button, just in case, for some reason, you have something against long, high-scoring words.)
Be prepared to be astounded by words you never even knew existed, or long words that you probably never would have found on your own. Show Best Word is great for those levels that ask you to score a lot of points, or to make 8 and 9 letter words.
In order to avoid wasting this powerful booster, you must have at least 5 letters available in order to use it.