Hand to: Robert Baratheon
Jon Arryn was a mentor to Robert (and Ned) before the events of Game of Thrones started, but the controversy surrounding his sudden death meant that his actions when he served as Hand were closely examined. Ned Stark investigated what Jon Arryn had been up to just before his death and came to the shocking conclusion that Robert's children were actually Cersei and Jaime's children — and Jon Arryn had figured it out as well.
In season four, Lysa "Crazytown" Arryn, Jon's widow (pictured here in lieu of her dead husband), revealed that she had poisoned her husband at the urging of Petyr Baelish and essentially put many of the events of Game of Thrones in motion. Sorry, Jon. When you've got Littlefinger against you, you don't really stand a chance.
