The AP story will probably get the most traction but only because it directly effects the press.
Benghazi is the worst of the bunch because it's the one Obama is most directly involved in and most clearly shows the disconnect he has between politics and national security.
F&F is more a Holder issue than an Obama one.
The IRS story, like F&F, is going to be hard to tie back to Obama.
If I had to pick the absolute worst scandal it would be the exploitation of the Newtown incident to make a head on assault on individual liberty and flagrant disregard for the Constitution.