#1 Sami Zayn
It's quite possible that Sami Zayn will make his return either before or right after WrestleMania 35. If you ask me, he'll return during the Superstar Shakeup, where he'll be drafted to the Raw brand.
When he left due to an injury, Zayn was one of the top heels on the roster, and I'd expect him to keep that role when he returns. Sure, he'll get a great reaction on the first night, but after that, he should continue to be a heel, as it fits him a lot better.
Obviously, Zayn is extremely talented, and he's more than capable of having great matches with just about anyone. He'd do the same with Roman Reigns.
It's also possible that both Roman Reigns and Sami Zayn take part in this year's Money in the Bank ladder match, which will take place on May 19. If that ends up being the case, then I expect Reigns to win that match.