Just start with Kai Hansen. He is the man when it comes to European power metal. He created Helloween and was their main songwriter on their 80s albums. He created Gamma Ray after he left Helloween. He has been on numerous Blind Guardian albums. He was in Iron Savior in the late 90s. He's been heavily involved in the European metal scene as he was also on Tobias Sammet's Avantasia and on the album Temple Of Shadows by Angra. Plus I think Gamma Ray is one of the best bands ever checkout the album Powerplant.