Matthew 19:16-17
16 And, behold, one came and said unto him, Good Master, what good thing shall I do, that I may have eternal life?
17 And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.
In Matthew 19:16-17 (in some manuscripts), Mark 10:17-18, and Luke 18:18-19, Jesus says that only God is good. Yet, Jesus is sinless (2 Corinthians 5:21, Hebrews 4:15: 1 Peter 2:21-22: 1 John 3:5), and calls Himself the "good shepherd" (John 10:11, 10:14).
In this account, Jesus is stating that the ultimate source of goodness is God, and that God is good in some underived sense that belongs to Him alone. Jesus also explicitly distinguishes Himself from God in this passage. To the audience hearing Him speak, this would have been a rather straightforward statement that carried with it the implication that Jesus definitely was not God. Consequently, this has been used as a Unitarian proof text.
Many Trinitarians interpret Jesus's words here as Him hinting that He is in fact God, using rhetoric. Even though the surface level of Jesus's statement appears to deny being "good" in the sense that He is referring to, the Trinitarian reading asserts that He actually was "good" in this sense, and He is asking the question rhetorically for those who are informed to reflect on later, and conclude that He is in fact God.
However, consider this from a big picture perspective, and observe as a reader of the New Testament that it is not the habit of God in Scripture to teach a major, fundamental doctrine via the use of what would be an ambiguous rhetorical question. The synoptic Gospels in which this account appears are (as lamented by many Trinitarians) bereft of verses which can be argued to teach that Jesus is God. Yet, they contain many verses in which Jesus is explicitly distinguished from God, without qualification. If a person were only to have these Gospels, they would never conclude that Jesus is God. If the authors of these Gospels intended to communicate that teaching, they would do so explicitly. And, if the passage in question was the principle place in which they intended to teach it, it would be emphasized, or clarified. Instead, it is left as it is, because Jesus is not saying that He is absolutely "good" in some sense that only belongs to God, and therefore God, but rather, He is teaching the opposite. It is a statement of humility consistent with His broader message (Matthew 23:12, Luke 14:11, 18:14), and in praise and exaltation of God, above Himself.