My research reinforces my opinion that reunion albums do not hold a huge amount of promise. For my tastes, the only bands whose reunion album is their best work are Mekong Delta, Sculptured, and Teramaze. Also Satan if you don't consider their masterpiece under another name, Pariah - Blaze of Obscurity. Exumer, Gorguts, Hell, and Mr. Big's reunion albums are also my favorite releases by those bands, but I did not care for their earlier work.
I added the list of "return to form" albums to acknowledge bands I like who were active throughout their career or reunited and released a crappy album first, but they managed to surprise me with their first enjoyable album in a long period of time. These are also comebacks by a different definition.
Reunion album that is similar in quality to the band's best work
Reunion album / Previous album
Artillery - When Death Comes (2009) / B.A.C.K. (1999)
Disaffected - Rebirth (2012) / Vast (1995)
Ira Deum - Demoratoria Irae - Back in Dark Ages (2011) / Reincarnation of the Immortal Evil (1995)
Maelstrom - It Was Predestined EP (2008) / This Battle to Make History, Yet History Never Comes Demo (1991)
Mekong Delta - Lurking Fear (2007) / Pictures at an Exhibition (1996)
Satan - Life Sentence (2013) / Court in the Act (1983)
Sculptured - Embodiment: Collapsing under the Weight of God (2008) / Apollo Ends (2000)
Teramaze - Anhedonia (2012) / Not the Criminal EP (2001)
Warlord - The Holy Empire (2013) / And the Cannons of Destruction Have Begun... (1984)
Worthwhile reunion album that does not measure up to the band's best work
All Too Human - Juggernaut (2012) / Entropy (2002)
Atheist - Jupiter (2010) / Elements (1993)
Believer - Gabriel (2009) / Dimensions (1993)
Black Sabbath - Dehumanizer (1992) / Mob Rules (1981)
Carcass - Surgical Steel (2013) / Swansong (1996)
Cynic - Traced in Air (2008) / Focus (1993)
Exodus - Tempo of the Damned (2004) / Force of Habit (1992)
Faith or Fear - Titanium (2012) / Punishment Area (1989)
Forbidden - Omega Wave (2010) / Green (1997)
Forte - Unholy War (2012) / Rise Above (1999)
Heathen - The Evolution of Chaos (2009) / Victims of Deception (1991)
Heaven's Cry - Wheels of Impermanence (2012) / Primal Power Addiction (2002)
Helstar - The King of Hell (2008) / Multiples of Black (1995)
Leatherwolf - World Asylum (2006) / Street Ready (1989)
Mercyful Fate - In the Shadows (1993) / Don't Break the Oath (1984)
New Eden - Solving for X (2012) / Stagnant Progression (2003)
Night in Gales - Five Scars (2011) / Necrodynamic (2001)
Pariah - Unity (1998) / Blaze of Obscurity (1989)
Sacred Warrior - Waiting in Darkness (2013) / Obsessions (1991)
Van Halen - A Different Kind of Truth (2012) / 1984 (1984)
Cool reunion album, but I did not care for their earlier work
Exumer - Fire & Damnation (2012) / Rising from the Sea (1987)
Gorguts - Colored Sands (2013) / From Wisdom to Hate (2001)
Hell - Human Remains (2011) / Demo 1986 (1986)
Mr. Big - What If... (2010) / Actual Size (2001)
Return to form / First album I liked from the band in a long time
Accept - Blood of the Nations (2010)
Accuser - Dependent Domination (2011)
Agent Steel Alienigma (2007)
Anthrax - Worship Music (2011)
Candlemass - Death Magic Doom (2009)
Death Angel - Relentless Retribution (2010)
Destruction - All Hell Breaks Loose (2000)
Flotsam and Jetsam - The Cold (2010)
Hellbastard - Sons of Bitches EP (2012)
Iron Maiden - Brave New World (2000)
Kreator - Violent Revolution (2001)
Machine Head - Through the Ashes of Empires (2003)
Megadeth - United Abominations (2007)
Mortal Sin - Psychology of Death (2011)
Motley Crue - Saints of Los Angeles (2008)
Onslaught - Sounds of Violence (2011)
Overkill - Ironbound (2010)
Ozzy Osbourne - Scream (2010)
Paradox - Electrify (2008)
Primus - Green Naugahyde (2011)
Rage - Soundchaser (2003)
Ratt - Infestation (2010)
Riot - Immortal Soul (2011)
Slayer - Christ Illusion (2006)
Tankard - Beast of Bourbon (2004)
The Dillinger Escape Plan - One of Us Is the Killer (2013)
The Mars Volta - Noctourniquet (2012)
Voivod - Target Earth (2013)
Did not like this band until this reunion album:
Anihilated - Scorched Earth Policy (2010)
Bitches Sin - The Rapture (2011)
Obsession - Order of Chaos (2012)
Satan's Host - By the Hands of the Devil (2011)
Vendetta - Feed the Extermination (2011)
I added the list of "return to form" albums to acknowledge bands I like who were active throughout their career or reunited and released a crappy album first, but they managed to surprise me with their first enjoyable album in a long period of time. These are also comebacks by a different definition.
Reunion album that is similar in quality to the band's best work
Reunion album / Previous album
Artillery - When Death Comes (2009) / B.A.C.K. (1999)
Disaffected - Rebirth (2012) / Vast (1995)
Ira Deum - Demoratoria Irae - Back in Dark Ages (2011) / Reincarnation of the Immortal Evil (1995)
Maelstrom - It Was Predestined EP (2008) / This Battle to Make History, Yet History Never Comes Demo (1991)
Mekong Delta - Lurking Fear (2007) / Pictures at an Exhibition (1996)
Satan - Life Sentence (2013) / Court in the Act (1983)
Sculptured - Embodiment: Collapsing under the Weight of God (2008) / Apollo Ends (2000)
Teramaze - Anhedonia (2012) / Not the Criminal EP (2001)
Warlord - The Holy Empire (2013) / And the Cannons of Destruction Have Begun... (1984)
Worthwhile reunion album that does not measure up to the band's best work
All Too Human - Juggernaut (2012) / Entropy (2002)
Atheist - Jupiter (2010) / Elements (1993)
Believer - Gabriel (2009) / Dimensions (1993)
Black Sabbath - Dehumanizer (1992) / Mob Rules (1981)
Carcass - Surgical Steel (2013) / Swansong (1996)
Cynic - Traced in Air (2008) / Focus (1993)
Exodus - Tempo of the Damned (2004) / Force of Habit (1992)
Faith or Fear - Titanium (2012) / Punishment Area (1989)
Forbidden - Omega Wave (2010) / Green (1997)
Forte - Unholy War (2012) / Rise Above (1999)
Heathen - The Evolution of Chaos (2009) / Victims of Deception (1991)
Heaven's Cry - Wheels of Impermanence (2012) / Primal Power Addiction (2002)
Helstar - The King of Hell (2008) / Multiples of Black (1995)
Leatherwolf - World Asylum (2006) / Street Ready (1989)
Mercyful Fate - In the Shadows (1993) / Don't Break the Oath (1984)
New Eden - Solving for X (2012) / Stagnant Progression (2003)
Night in Gales - Five Scars (2011) / Necrodynamic (2001)
Pariah - Unity (1998) / Blaze of Obscurity (1989)
Sacred Warrior - Waiting in Darkness (2013) / Obsessions (1991)
Van Halen - A Different Kind of Truth (2012) / 1984 (1984)
Cool reunion album, but I did not care for their earlier work
Exumer - Fire & Damnation (2012) / Rising from the Sea (1987)
Gorguts - Colored Sands (2013) / From Wisdom to Hate (2001)
Hell - Human Remains (2011) / Demo 1986 (1986)
Mr. Big - What If... (2010) / Actual Size (2001)
Return to form / First album I liked from the band in a long time
Accept - Blood of the Nations (2010)
Accuser - Dependent Domination (2011)
Agent Steel Alienigma (2007)
Anthrax - Worship Music (2011)
Candlemass - Death Magic Doom (2009)
Death Angel - Relentless Retribution (2010)
Destruction - All Hell Breaks Loose (2000)
Flotsam and Jetsam - The Cold (2010)
Hellbastard - Sons of Bitches EP (2012)
Iron Maiden - Brave New World (2000)
Kreator - Violent Revolution (2001)
Machine Head - Through the Ashes of Empires (2003)
Megadeth - United Abominations (2007)
Mortal Sin - Psychology of Death (2011)
Motley Crue - Saints of Los Angeles (2008)
Onslaught - Sounds of Violence (2011)
Overkill - Ironbound (2010)
Ozzy Osbourne - Scream (2010)
Paradox - Electrify (2008)
Primus - Green Naugahyde (2011)
Rage - Soundchaser (2003)
Ratt - Infestation (2010)
Riot - Immortal Soul (2011)
Slayer - Christ Illusion (2006)
Tankard - Beast of Bourbon (2004)
The Dillinger Escape Plan - One of Us Is the Killer (2013)
The Mars Volta - Noctourniquet (2012)
Voivod - Target Earth (2013)
Did not like this band until this reunion album:
Anihilated - Scorched Earth Policy (2010)
Bitches Sin - The Rapture (2011)
Obsession - Order of Chaos (2012)
Satan's Host - By the Hands of the Devil (2011)
Vendetta - Feed the Extermination (2011)