Objection 1. It seems that God is not the supreme good. For the supreme good adds something to good; otherwise it would belong to every good. But everything which is an addition to anything else is a compound thing: therefore the supreme good is a compound. But God is supremely simple; as was shown above (Question 3, Article 7). Therefore God is not the supreme good.
Objection 2. Further, "Good is what all desire," as the Philosopher says (Ethic. i, 1). Now what all desire is nothing but God, Who is the end of all things: therefore there is no other good but God. This appears also from what is said (Luke 18:19): "None is good but God alone." But we use the word supreme in comparison with others, as e.g. supreme heat is used in comparison with all other heats. Therefore God cannot be called the supreme good.
Objection 3. Further, supreme implies comparison. But things not in the same genus are not comparable; as, sweetness is not properly greater or less than a line. Therefore, since God is not in the same genus as other good things, as appears above (3, 5; 4, 3) it seems that God cannot be called the supreme good in relation to others.
On the contrary, Augustine says (De Trin. ii) that, the Trinity of the divine persons is "the supreme good, discerned by purified minds."
I answer that, God is the supreme good simply, and not only as existing in any genus or order of things. For good is attributed to God, as was said in the preceding article, inasmuch as all desired perfections flow from Him as from the first cause. They do not, however, flow from Him as from a univocal agent, as shown above (Question 4, Article 2); but as from an agent which does not agree with its effects either in species or genus. Now the likeness of an effect in the univocal cause is found uniformly; but in the equivocal cause it is found more excellently, as, heat is in the sun more excellently than it is in fire. Therefore as good is in God as in the first, but not the univocal, cause of all things, it must be in Him in a most excellent way; and therefore He is called the supreme good.
Reply to Objection 1. The supreme good does not add to good any absolute thing, but only a relation. Now a relation of God to creatures, is not a reality in God, but in the creature; for it is in God in our idea only: as, what is knowable is so called with relation to knowledge, not that it depends on knowledge, but because knowledge depends on it. Thus it is not necessary that there should be composition in the supreme good, but only that other things are deficient in comparison with it.
Reply to Objection 2. When we say that good is what all desire, it is not to be understood that every kind of good thing is desired by all; but that whatever is desired has the nature of good. And when it is said, "None is good but God alone," this is to be understood of essential goodness, as will be explained in the next article.
Reply to Objection 3. Things not of the same genus are in no way comparable to each other if indeed they are in different genera. Now we say that God is not in the same genus with other good things; not that He is any other genus, but that He is outside genus, and is the principle of every genus; and thus He is compared to others by excess, and it is this kind of comparison the supreme good implies.
Source: Summa Theologica (New Advent)