#1 vs. Becky Lynch

Well, obviously.
This is the biggest WWE match that is yet to happen since they began building to it during the fall of 2018. Ronda Rousey and Becky Lynch have had the most personal feud not to feature a one-on-one match.
The two were set to face each other at Survivor Series before Nia Jax broke Becky's nose. This eventually led to a Royal Rumble victory for The Man, who challenged Rousey to a RAW Women's Title match at WrestleMania.
As we all know, Charlotte Flair entered herself into the feud and headlined the Show of Shows along with the two of them. That match saw Becky Lynch pin Ronda Rousey to win the RAW and SmackDown Women's Championships, even though Rousey's shoulder was off the mat.
Even without the fact that they never faced each other one-on-one, that controversial finish effectively confirms Becky Lynch vs Ronda Rousey as a match that will happen, if the latter does return to WWE.
The two have cut some scathing promos on each other, hitting on reality a bit too much. It may be possible that they actually don't like each other. Either way, it makes for a fascinating story and compelling TV.
After over a year as the champ, The Man could be dethroned by the Baddest Woman on the Planet herself.