#1 Seth Rollins

While it may or may not depend on if he takes the Universal Championship off of Brock Lesnar, Rollins is the biggest star in the company and has been pushed like it over the last six months.
Roman Reigns will eventually get back there, and maybe even the boos from the fans once he returns to Superman status, but Rollins is loved whether he's a face or a heel.
Look who just shut down all of Tony Khan's critics! More HERE.
WWE cannot afford to have a wrestler portrayed as a face getting booed once it moves over to Fox.
They need to keep the ratings strong and putting the Universal Championship on Rollins ensures that he'll be on the show weekly as well as that every match he'll be in will be great.
If WWE needs to up the ratings for SmackDown, they will have to put one of the biggest stars in the company on the show and the main champs of each show might just switch brands this year.
Daniel Bryan fits on either show so Rollins might not necessarily get moved to SmackDown. Brock Lesnar was advertised for SmackDown in the fall, so if Lesnar is considered one of the biggest stars in the company, then the man who should beat him in April should also be.