If you think Zadina is one of the biggest busts in NHL history you must be new to following the NHL.
End of the day, players picked around 6 overall bust a lot more than fanbases picking in that range would like to think. It's not like Kotkaniemi and Barret Hayton have exactly taken the league by storm. While objectively a worse player, guys like Dylan Strome, Zacha and Adam Larsson had massive hype attatched to them and haven't live up to it. Michael Dal Colle is playing in Finland. Griffin Reinhart was playing in Northern Ireland last year.
We want to talk biggest busts of all-time. Look at guys who went one or two and came nowhere close to living up to expectations like Alexander Daigle, Patrik Stefan, Andrei Zyuzin, etc.