Well I'm going to throw my hands up in defeat and declare it here, now. I didn't feel there was a single best album of 2015, so I will call ties in Metal and Rock.
Best Metal:
Tie between Blind Guardian - Beyond the Red Mirror and Symphony X - Underworld.
Best Rock:
3-way between Beardfish - +4626-Comfortzone, Riverside - Love, Fear and the Time Machine, and Subsignal - The Beacons of Somewhere Sometime.
Other standouts include:
Caligula's Horse - Bloom
Dead Letter Circus - Aesthesis
Transience - Temple (already declared my Best Debut)
Kingcrow - Eidos
Clutch - Psychic Warfare (not prog but killer Hard Rock that must be mentioned)
Soilwork - The Ride Majestic
Queensryche - Condition Human (I was unsure if I would dig the post-Tate version of the band, but this one sold it to me good.)