#1 John Cena
Brock Lesnar and John Cena had an amazing rivalry in the past. They have beat the hell out of one another, and, have always been outstanding competitors inside the ring. Therefore, a match at WrestleMania 35 between John Cena and Brock Lesnar would be a good idea. It could be a main event worthy match, and, would be good from WWE's point of view as well.
Both John Cena and Brock Lesnar won't be hovering around for many years in the WWE. A competition between the duo is always a treat to watch and, if the creative team of WWE plans a good feud, it shall be an icing on the top.
The match should be conducted under special stipulations, and, it could be a career vs career match as well. The loser will quit WWE (kayfabe) and that should garner heavy responses from the crowd.