Aaron Gordon
There are a number of power forwards who are potentially acquirable in free agency this summer, although the big names are restricted free agents and come with the caveat that they might need to be signed-and-traded to be had.
That being said, any one of Jabari Parker, Aaron Gordon, and Julius Randle would be significant improvements at power forward over what the team has right now and each would make a significant impact on the team’s ability to win immediately.
Of those three, my personal preference is Aaron Gordon as his athleticism and developing 3-point shot makes him a younger Blake Griffin, someone I believed that the Suns should have targeted last season (and did, to a degree). Gordon would step in as the team’s second 20-point scorer and at 23 would be a piece of the future core for a long time.
Gordon though is almost certainly going to have any restricted contract offer matched by the Orlando Magic, unless he publicly demands that the franchise does not match it, and thus will be difficult to acquire. However, if the Suns are serious about upgrading the power forward position, doing everything they can to steal Gordon away from Orlando should be a priority.