#1 John Cena
John Cena is the most deserving candidate to win the Grand Slam. He has won almost every possible championship and has defeated every wrestler he came across. The only hurdle between him and the Grand Slam is the Intercontinental Championship.
It is a shame that a wrestler like Cena has never been booked in an Intercontinental Championship match. He won't be hovering around for a long time in the WWE, and he deserves to be crowned as a Grand Slam Champion. Therefore, WWE might consider a contest between Cena and Bobby Lashley for the Intercontinental Championship.
It is quite unfortunate that a veteran like John Cena has not won the Grand Slam. The best option would be to make him win the Intercontinental Championship at WrestleMania 35. It would be good for business, as fans would love to see John Cena as their next Intercontinental Champion.