Kostin seems great, but hadn't he dropped from that #1 spot because of his injury?
He was still listed as the #1 European Skater on Central Scouting's April (end of season) rankings, followed by Patterson (taken #5 to Vancouver), Andersson (#7 to NYR), and Heiskanen (#3 to Dallas).
Obviously the injury dropped him pretty significantly in the eyes of most NHL teams. It'll be interesting to see how he develops. If he can recover fully and reach his potential then it's an absolute steal of a pick.