#4 Roman Reigns

This one was a given. He's been the 'Golden Boy' of the WWE for the last four years so that wasn't going to change much this year. Although he had to take a detour due to sickness, he picked up where he left off as the Shield reformed to stomp out Baron Corbin, Drew McIntyre and Bobby Lashley.
Once that partnership ended again, he feuded with and defeated McIntyre at WrestleMania. The main question on a lot of people's minds is just how long it will be until he's thrust right back into title contention. Mr. McMahon might panic because of the drop in ratings and just have him challenge for it within the next month or so but that would be a mistake.
Something that might have turned some fans over the last few years was the constant push of the same superstars (Reigns, Flair, Rollins, Lesnar, Styles). While some of those wrestlers are more than deserving, it stagnates the product when no one new gets a run with one of the top titles.
Reigns is currently feuding with Elias and Shane McMahon, but any random superstar doesn't get to battle McMahon. The Miz, Styles and the Undertaker are all trusted vets who were able to get good storylines and/or matches out of the son of 'the Chairman'.
Whether he's trying to topple a McMahon or trying to reclaim a title at some point this year, Reigns is going to be one of the superstars who will continually dominate the entire company for the rest of this year and most of the next decade.