The first interpretation, that the sons of God are Seth's descendants, does not have solid support. At first, this interpretation appears as if it might fit. However, upon closer inspection it becomes evident that such an interpretation relies upon a proposition which is impossible to support— that Seth's descendants were especially righteous. To support this view, there would need to be Scriptural evidence that in the early parts of human history, Seth's descendants were typically righteous and that the descendants of all other men were typically unrighteous.
While it is certain from the genealogy in Genesis 5 that there were at least two righteous men in Seth's lineage (at least three if Lamech is included, Genesis 5:28), this still gives no evidence that all of Seth's descendants were generally righteous. In fact, the last verse of Genesis 4 states (emphasis added): Then men began to call upon the name of the LORD. (Genesis 4:26) In the one place in Scripture that refers to the general righteousness of man in the pre-flood world, the righteousness is not presented as a quality which typically belonged to Seth's descendants. Rather, the verse simply states that men (not the descendants of Seth) began to call upon the name of the LORD.
The second interpretation, that the sons of God are powerful kings, finds its support in that kings in the early parts of human history sometimes called themselves sons of God. This, I think, is a better interpretation than the view that the sons of God are descendants of Seth. However, like the first interpretation, this interpretation also lacks internal biblical evidence.
At first, the third interpretation, that the sons of God are fallen angels, might seem obscure. However, such an interpretation has strong internal biblical support. Every time the phrase sons of God appears in the Old Testament, it refers to angels (Job 1:6, 2:1, 38:7, Psalm 29:1, 89:7, Daniel 3:25). Of all of these passages, the ones which are most clear concerning the identity of the sons of God are the passages found in Job. Considering that most believe the book of Job was written in the general time period Genesis was written (probably before), this only adds to the evidence that the sons of God in Genesis are indeed angels. If the phrase sons of God could refer to angels in Job's time, it seems logical to conclude that the phrase could carry the same meaning in Moses' time.
Evidence for the interpretation that the sons of God are angels can also be found in the New Testament. The books 1 Peter, 2 Peter, and Jude all mention demonic activity which can be connected with the sons of God in Genesis 6.
Who Are the Sons of God? (Part 4) - Genesis 6:1-4
The Birth of Seth - Genesis 4:25
State of the World (2948 BC): Noah's Birth - Genesis 5:28
The Genealogy: Adam to Noah - Genesis 5
Summary of Genesis 5