This issue has been debated for several centuries among literary critics. In one sense, one could argue that Paradise Lost is not written in the form of either a tragedy or an heroic epic, and thus that there really are no "heroes" in the traditional sense in the work.
The protagonist of the poem is Lucifer, the brightest of all God's angels, who due to his disobedience to God was cast out of Heaven into Hell. Many critics consider Satan the hero of the poem, as the main theme of the poem is Satan's fall.
Although Jesus only play a minor role in the narrative, perhaps the most heroic act of the story is the decision of Jesus to sacrifice himself for humanity, but this only occupies a small part of the narration at the end of the book.
Although God is obviously the greatest and most perfect character in the story, because of what theologians term his impassibility (inability to suffer), God, being purely divine and not needing to struggle against human limitations, cannot really be considered a hero.