It will be difficult to pick one as the best superstar in WWE just as it will be difficult to answer who the best actor is in Hollywood. Not withstanding, there are some super stars in WWE that have made a name for themselves in this industry.
In no particular order, some of them are: AJ styles, Bret Hart'The Hitman', Andre The Giant, Shawn Micheals, Roman Reigns, John Cena, Hulk Hogan, The Undertaker, Steve Austin, The Rock, Randy Orton and Triple H.
It will be difficult to pinpoint who exactly is the biggest star of the WWE mainly because it depends on their fans. It should also be pointed out that some superstars are usually popular at one point in their career then booed the next before being well-liked again. If a choice have to be made, it has to be Undertaker.
The fact that all of his gimmicks through the years are embraced by their fans, plus the fact that he is highly respected in the industry - these things are proof that he is one of the biggest stars to ever grace the ring of WWE. Another great reason why he is the biggest superstar of WWE - he looks the part.