Some of the hacks of 2015 can be listed as worthy of mention - here goes - Ashley Madison, V tech, Vodafone, Talk Talk, JD Wetherspoon, Office of Personnel Management, Anthem, Premera, IRS, Slack, the FBI portal, Car Phone Warehouse, Samsung and Hilton. This does not include the long running media company hacks of live feeds by sites such as CricFree.
Without a clear definition of Hack it is very difficult to identify a winner. The OED defines hacker as below
This leaves a lot of room for manoeuvre and does not necessarily suggest that hacking breaks the law.
With that latitude my favourite is the hack of the Tesla Model S by Marc Rogers and Kevin Mahaffey - watch the video below.