#3 Finn Balor
Finn Balor put on a great match with Roman Reigns a few weeks ago on Monday Night Raw after Reigns beat Brock Lesnar for the Universal Championship at SummerSlam. At that point, it seemed that Balor was poised for a push but plans for that seem to have fallen away.
The unique thing about Balor at the moment is that one never knows what to expect with him. He could use his Demon King persona at any moment and this is something which Roman Reigns has not encountered before. Yes, Reigns has faced The Undertaker and won, but this was an Undertaker who has been far from his best and well out of his prime. The same cannot be said for Balor.
Reigns might be physically superior to Balor but the latter's style and agility would prove a lot for Reigns to handle and is something which he would need to adjust to. There is also the demon persona which Reigns has not faced and this is something which would make for a thrilling feud.