I would like to propose an Ontological Argument for the existence of God that St. Thomas himself might agree with. St. Thomas did not agree with St. Anslem's original ontological argument. However, I think in this form, St. Thomas might have accepted it. It's really quite simple. But, perhaps, someone can point out any flaws in it.
The Thomistic Ontological Argument for the Existence of God
1. God, by definition, is a maximally good being.
2. According to St. Thomas, Goodness is equivalent to Existence. (S.T. Part 1, Question 5, Art. 1) (I believe this is from Plato originally)
3. Therefore, a maximally good being (one who's essence is Goodness) must actually exist.
4. Therefore, God exists.
To elaborate on item 2 from a Platonic perspective (although St. Thomas uses an Aristotelian example in his S.T.), Goodness makes something what it is. A knife that is very dull and useless for cutting is not a good knife. In fact, if the knife is so bad at cutting, it would cease to be a knife. So, Goodness Itself defines existence. Note that this has nothing to do with something physically existing in the material world. For example, if a mathematical proof of Fermat's Last Theorem is a good proof, then that proof exists. However, this doesn't imply the proof has physical existence.