#2 The Undertaker
The Demon King avatar of Finn Balor is one of the mystical characters produced by the WWE Creative team in the modern era. While, the Undertaker character was (and still is) one of the best mythical on-screen WWE characters. Having a showdown between the two superstars would certainly break the internet.
Finn Balor and the Undertaker are two of the biggest names in wrestling industry and it is baffling why the WWE Creative team chose to waste Undertaker's Greatest Royal Rumble appearance in a casket match against Rusev.
I don't undervalue the Bulgarian Brute here, but Finn Balor is certainly a grander superstar than him and thus, it would have made more sense to see the Demon King and the Undertaker battle in a casket match. However, this match can still happen in the coming year which would seemingly the Deadman's last in the WWE.