For me it's Watershed. It's so disgustingly heavy but also so amazingly delicate at times.
Opeth have always had beautiful moments in their songs. The line from Closure when Mikael sings 'in the rays of the sun I am longing for the darkness', the solo in Windowpane, the intro guitar to Dirge for November, Patterns in the Ivy, Isolation Years etc. But never have Opeth ever made me cry, yet when I first heard Coil in my room with the lights out and my expensive headphones it made me feel the way that only the top bands in my life make me feel. The end riff to Heir Apparent was something magical, with Hessian Peel having my favourite Opeth riff ever in it.
I never thought I'd say it but I'm glad they had the line up change. I've listened to Watershed every day at least 4 times for the last month and a half and everytime it gets more special.
Ghost Reveries had the great moments, but Watershed is just a more mature band, more sure of themselves than before (or maybe less), and I love it. For me Watershed wins hands down, but that's not to say Ghost Reveries isn't a good album too. It's just that Watershed is also an indication of where one of my favourite bands are going, and I love it. I can have a whole album of songs like Burden to be honest, all the fans saying they don't like that sort of stuff didn't get Opeth to begin with, so fuck 'em.