Beauty in God's eye
The short answer is: Himself. The long answer is: Anything that reflects His own glory. The negative being nothing that does not reflect His glory. So, when God sees creation, He says "It is good", not because creation is somehow intrinsically good on its own, but because "the heavens declare the glory of God and the sky above proclaims His handiwork". The reason God loves nature is because it displays, it manifests His power and wisdom and goodness and knowledge. Conversely, when God sees a man by himself, he says "it is not good for man to be alone", not because it is for some reason bad not to have a date on Friday night, but because there's something about the glory of God that is not displayed when a man is alone. And when He looks at a covenant marriage between a man and a woman, where a man loves his wife and his wife submits to her husband, He finds it beautiful because in it is a glorious display of the gracious love of His Son for His Bride and Her joy in Him. Anything in this world that displays some aspect of the glory of God, God will find beautiful. And anything in this world that hides the glory of God (which is ultimately what sin is... a falling short of the glory of God), God will condemn as evil and ugly.
The implication of this is huge for Christians. We should always have an eye to the glory of God in everything that we appreciate as beautiful and good in this world... a crisp autumn day, a hug from a friend, a tall glass of orange juice, an encouraging conversation, a game of football, and on and on... the question for the Christian in all these things is, "What is it that is good and beautiful about what I am enjoying?" If it is an aspect of the glory of God, then you are on the right track! If it is something else, then it is idolatry. And the same would go for the opposite... why is lying evil? Why is cowardice evil? Why is dishonoring your parents evil? Why is pornography evil? Why is being a lazy husband or a domineering wife evil?... the ultimate answer for the Christian must be because these things hide the beauty and glory and worth of God. The veiling of true beauty (ie: the glory of God) is the greatest reason why it should grieve us that we have sin in our lives and this "hiddenness" of the glory of God is what we must savagely fight against with all our strength.