Objection 1. It seems that to be essentially good does not belong to God alone. For as "one" is convertible with "being," so is "good"; as we said above (Question 5, Article 1). But every being is one essentially, as appears from the Philosopher (Metaph. iv); therefore every being is good essentially.
Objection 2. Further, if good is what all things desire, since being itself is desired by all, then the being of each thing is its good. But everything is a being essentially; therefore every being is good essentially.
Objection 3. Further, everything is good by its own goodness. Therefore if there is anything which is not good essentially, it is necessary to say that its goodness is not its own essence. Therefore its goodness, since it is a being, must be good; and if it is good by some other goodness, the same question applies to that goodness also; therefore we must either proceed to infinity, or come to some goodness which is not good by any other goodness. Therefore the first supposition holds good. Therefore everything is good essentially.
On the contrary, Boethius says (De Hebdom.), that "all things but God are good by participation." Therefore they are not good essentially.
I answer that, God alone is good essentially. For everything is called good according to its perfection. Now perfection of a thing is threefold: first, according to the constitution of its own being; secondly, in respect of any accidents being added as necessary for its perfect operation; thirdly, perfection consists in the attaining to something else as the end. Thus, for instance, the first perfection of fire consists in its existence, which it has through its own substantial form; its secondary perfection consists in heat, lightness and dryness, and the like; its third perfection is to rest in its own place. This triple perfection belongs to no creature by its own essence; it belongs to God only, in Whom alone essence is existence; in Whom there are no accidents; since whatever belongs to others accidentally belongs to Him essentially; as, to be powerful, wise and the like, as appears from what is stated above (Question 3, Article 6); and He is not directed to anything else as to an end, but is Himself the last end of all things. Hence it is manifest that God alone has every kind of perfection by His own essence; therefore He Himself alone is good essentially.
Reply to Objection 1. "One" does not include the idea of perfection, but only of indivision, which belongs to everything according to its own essence. Now the essences of simple things are undivided both actually and potentially, but the essences of compounds are undivided only actually; and therefore everything must be one essentially, but not good essentially, as was shown above.
Reply to Objection 2. Although everything is good in that it has being, yet the essence of a creature is not very being; and therefore it does not follow that a creature is good essentially.
Reply to Objection 3. The goodness of a creature is not its very essence, but something superadded; it is either its existence, or some added perfection, or the order to its end. Still, the goodness itself thus added is good, just as it is being. But for this reason is it called being because by it something has being, not because it itself has being through something else: hence for this reason is it called good because by it something is good, and not because it itself has some other goodness whereby it is good.
Source: Summa Theologica (New Advent)