#1 John Cena

The last time these two men faced each other, it was voted as the 'Match of the Year'. Cena won that encounter and became the WWE Champion for the sixteenth time.
All records are meant to be broken. Cena has the opportunity to become a 17-time WWE Champion if he defeats Styles for the WWE Championship at the Royal Rumble. He would effectively beat Ric Flair's long-standing record of 16 reigns, in that case.
Don't forget that John Cena is a free agent. While he was last seen on RAW, he could just as well move to SmackDown Live, for a program with Styles.
The coming weeks should reveal who Styles' opponent is. Whoever it is, the outcome will be 'Phenomenal'. Styles is a performer who's always delivering the goods. He could wrestle a broomstick, and it'd still be great!