Here's what the Logos commentary says (abbreviating). They say there are currently three possibilities: supernatural beings ("angels"), leaders, and godly men, "the descendants of Seth as opposed to the godless descendants of Cain."
"The “angel” interpretation is at once the oldest view and that of most modern commentators. It is assumed in the earliest Jewish exegesis (e.g., the books of 1 Enoch 6:2ff; Jubilees 5:1), LXX, Philo De Gigant 2:358), Josephus (Ant. 1.31) and the Dead Sea Scrolls (1QapGen 2:1; CD 2:17–19). The NT (2 Pet 2:4, Jude 6, 7) and the earliest Christian writers (e.g., Justin, Irenaeus, Clement of Alexandria, Tertullian, Origen) also take this line.
"Modern scholars who accept this view advance three main reasons for supporting it. First, elsewhere in the OT (e.g., Ps 29:1, Job 1:6) “sons of God” refers to heavenly, godlike creatures. Second, in 6:1–4 the contrast is between “the sons of the gods” on the one hand and “the daughters of man” on the other. The alternative interpretations presuppose that what Gen 6 really meant was that “the sons of some men” married “the daughters of other men.” The present phrase “sons of God” is, to say the least, an obscure way of expressing such an idea. It is made the more implausible by 6:1 where “man” refers to all mankind. It is natural to assume that in v 2 “daughters of man” has an equally broad reference, not a specific section of the human race. Finally, it is pointed out that in Ugaritic literature “sons of God” refers to members of the divine pantheon, and it is likely that Genesis is using the phrase in a similar sense."