I was gonna come in to say I loved March or Die, despite everyone not...but it looks like I did that at the time already, so...
This wouldn't have counted in 2015, but I guess now my least favorite is Aftershock, although it's still a great album. But I listen to it probably the least of any of their releases, so if I have to pick one, that's a good enough reason to go on.
Edit: Nevermind, looks like Aftershock came out in 2013, so it woulda counted in 2015. WTF--6 years ago? It feels so recent to me.