Let’s be fair to James… I’ll use post-war seventeen-year-old Harry vs James right before his death. Had I used Harry of the last chapter of the last book, he would have had years of advantage on James, nevermind the fact that he becomes head of Aurors.
On one side, we have James, he has four years of advantage on Harry. He was said to have dueled three times Voldemort and survived on The Prophecy, but we don’t know how many people were fighting along with him. We hear from most of his old teachers he was very bright. He’s said to be an extremely talented quidditch player, which would give him good reflexes.
On the other side, we have Harry. Harry has countless experiences with war and battle, he’s exceptionally good at defense against the dark arts. Although James is slightly older than him, Harry probably makes up for the difference with the D.A. and his encounters with Death Eaters. Harry is also an exceptional seeker, with great reflexes
I would say Harry would win. I’m assuming James is not nearly as powerful as big Death Eaters like Bellatrix or Rodulphus as he was just fresh out of school, did not know about the Dark Arts, and has spent the last year hiding instead of getting some hands-on experience. Seeing as Harry managed to not die when faced with Death Eaters, sometimes while protecting other people or things, I would say James doesn’t stand a chance.