Kucherov's all-time level playoff run just weighs more than regular season greatness, so I would put him in the conversation for best player in the game right beside McDavid, or right behind him or slightly ahead of him.
To frame it another way. When McDavid is going off on the powerplay for a 4 point night against the Vancouver Canucks, you can sit back and say "well, let's see him do this in the playoffs when it really matters." When Kucherov is lifting the cup, you don't say "well, you've won a Stanley Cup and dominated the playoffs, but let's see you do this on a Tuesday night in November."