#2 Styles is the man to make him look his best at this point

AJ Styles is considered to be one of the best performers of this generation. You could even go further and make the argument that he's one of the best of all time. Regardless of where you stand in that debate, his talent is undeniable.
It took him a while to get to the WWE, but it didn't take long for creative to realize how special he was, even at the tail end of his career. He's over 40 but still puts forth great matches.
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That's precisely why Styles is the one facing Undertaker at WM36. 'Taker wanted to work with Styles because he considers him a "modern-day Shawn Michaels." That's immense praise to be picked by one of the best of all time because he is reminiscent of another best of all time. The Phenomenal One is such a great in-ring worker that he'd be able to carry anyone, including a 55-year old Phenom, to a great match.
There are a few others on the roster that could do the same - Cesaro, Dolph Ziggler, Daniel Bryan - but who don't have as much star power as Styles. Bryan is a big name but isn't as similar to Michaels as Styles is. The Undertaker might still come back for a few more matches, but few should be as good as his bout with the former Impact Champion.