Zadorov has had his fair share of matchups against MacKinnon in his career, but McDavid has been a far greater rival from his time in the Battle of Alberta. There's probably no chance that he would recognize one of his rivals as the best player in the world, and instead had to side with MacKinnon. Zadorov was also a teammate in Colorado before being dealt to the Flames.
Mackinnon is having an MVP caliber season this year with an incredible 41 goals and 113 points in 66 games, but that doesn't make him the best player in the world. Zadorov's reasoning was that MacKinnon has proven himself to be a winner with the Avlanche Stanley Cup victory two years ago. On the other hand, in what's felt like a down year offensively for McDavid, he's on pace to become just the fourth player in NHL history to score 100 assists in a year.
MacKinnon has the edge in playoff success so far in his career, but Connor McDavid will undoubtedly win a Stanley Cup in his career - he's simply too good not to. It could come as early as this year, and if the Oilers do win the Cup, McDavid will unquestionably be the best player in the league with that hardware.