Who is "next in power and next in crime?" in Paradise Lost?

That would be Beelzebub, the Lord of the Flies himself. Although Beelzebub is often a name given to the Devil, in Paradise Lost he's actually Satan's second in command. That's what Milton means by "next in power." Beelzebub's also "next in crime" in that his crimes, though not as great as Satan's, are still pretty great all the same. (That's great as in "huge," rather than great as in "wonderful.")
During the famous debate among the fallen angels in Pandemonium, the wicked Beelzebub gives a powerful speech in which he argues for a new strategy in getting back at God. Instead of launching another direct assault on Heaven, the fallen angels should turn their attention towards corrupting Man, God's proudest creation. Either the forces of darkness can lay waste to God's earthly kingdom, or even better, seduce humankind into joining them in their struggle against the Almighty. It is the latter course of action that Satan will pursue when he makes his way to the Garden of Eden and corrupts Adam and Eve by persuading them to eat from the Tree of Knowledge.