One Decade, One Song - Antestor

90's - Kongsblod
00's - Via Dolorosa
10's - Unchained

Since the 90s or 00s (depending on whether you consider Martyrium a 90s or 00s album) is the only decade with more than one album, I might as well just list my favorite per album:

Martyrium: Inmost Fear
The Return of the Black Death: Battlefield
The Forsaken: Via Dolorosa
Omen: All Towers Must Fall

Dang that's a tough one.  I'll use CW's strategy.

Martyrium: Inmost Fear, though Martyrium is a verrrrry close second
The Return of the Black Death: Kongsblod
The Forsaken: Via Dolorosa or The Crown I Carry, depending on the day.
Omen: Benighted