#4 The Undertaker

The Undertaker wasn't part of this year's WrestleMania for the first time in almost two decades, but as of yet, The Deadman is yet to reveal whether or not he is officially retired.
Undertaker seemingly retired after losing to Roman Reigns at WrestleMania 33 but returned to face John Cena at WrestleMania 34. This year he skipped WrestleMania and instead made an appearance on the RAW After WrestleMania.
WrestleMania is Undertaker's playground. He's the owner of the longest undefeated streak in the history of the show, has easily the best record (22-2), and without him, this past year's WrestleMania felt like it was missing something.
Undertaker's career in WWE will reach 30 years next year and the perfect final bow for The Deadman could see him face The Fiend at WrestleMania and pass the torch on to the man who has been dubbed this generation's Undertaker.
WWE has plenty of time to set up this match for WrestleMania with the show still five months away and the likelihood that Daniel Bryan's feud with The Fiend will come to an end at The Royal Rumble. One thing is for sure, when Undertaker does decide to walk away from the ring, the biggest show of the year will never be the same again.