Easily Subban, Mike Green as a defenseman was a mirage. Basically he was horrible defensively but a solid puck mover and elite offensively benefitting greatly from the high octane offense/PP around him. As short lived as it was, Subban was a top tier defenseman for those couple years.
The Capitals, mainly George McPhee's, biggest blunder was mistaking Mike Green for a 1A dman and not surrounding him with some sort of stay at home, shut down guy. At best he was a second pairing guy and powerplay point specialist.