#1 Drew McIntyre

AJ Styles has had a Hall of Fame career even in his short time in WWE, and in the final few years in the ring, he will want to put over up-and-coming Champions like Drew McIntyre. For a babyface to get the love and adulation of fans, he will need to be excellent in the ring, have good mic skills, as well as a heel who can bring a lot of heat on him.
McIntyre has that now in his feud with Seth Rollins, and he will have it when he eventually feuds with former WWE Champion AJ Styles. Drew McIntyre and AJ Styles haven't had a singles feud in WWE. Styles did, though, play a part in building McIntyre's momentum heading into this year's Royal Rumble, when the current WWE Champion defeated Styles and another veteran, Randy Orton, in a triple threat match on RAW.
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The perfect set-up for this feud would begin with Styles winning the men's Money in the Bank Ladder Match, and then cashing it on Drew McIntyre sometime down the road.