My money is on Hank getting killed.
Hank isn't my top bet though, I feel he's too integral to the whole subverted plot of Walt dodging DEA.
Skylar seems safe because I'm sure Walt would bolt or go mental if he lost her.
That leaves Gus and Mike as primo targets, you have to judge how valuable Mike is to the show? Gus runs the majority of the drug business so I can't see it operating without him and there doesn't seem a worthy replacement.
So I'm gonna say goodbye Mike.
Walt Jr is on another level of options, that would be dark........