#2 Rey Mysterio should win because the product really needs a shake-up
Why should Rey Mysterio defeat Roman Reigns at Hell in a Cell? Because the good old days when WWE was unpredictable are long gone. Everyone expects Roman Reigns to pick up the victory in this contest, but maybe there's interference by Jimmy Uso that leads to the upset. Rey Mysterio winning, by any means, would just shake things up!
Nobody likes a story where you know what happens on the last page. Unpredictability and surprises make wrestling significantly more fun.
#3 Roman Reigns should win because there are more exciting opponents on the horizon
There are rumors doing the rounds that Roman Reigns vs. John Cena is on the horizon. There's also talk of a Brock Lesnar return, and if that happens, he could very well come for the current Universal Champion instead of Bobby Lashley.
Of course, Roman Reigns vs. Jimmy Uso is also a road we can travel down. Rey Mysterio, as legendary as he is, doesn't have the star power for John Cena's return match.