#4 Baron Corbin
Baron Corbin is doing a good job as a heel but he just doesn't seem to get over with the fans. The former Raw GM-Elect is booed in most of his matches. While it is normal for a heel to get that sort of heat from the fans, Corbin just doesn't connect with the fans.
Vince McMahon, however, seems to be a big fan of the heel and has been pushing him a lot recently. He was made the active Raw GM and within months, was involved in the WWE Universal Title picture as well. While he didn't win the title, he was given multiple matches against the champion, Seth Rollins.
Corbin has a long way to go before the fans get on his side, and for a character like his, it would be better to remain a heel as turning face might ruin things for him.