Caporegime
Off the top of my sleepy head, Button dominated the first half of his 2009 championship winning season thanks to the Brawn double diffuser, but once other teams like Redbull caught on he was typically finishing ~6th.
It was quite the drop off, but although Jenson didn't win another race, Barrichello won twice, and Jenson got two more podiums - but, yeah, probably the best comparison.