I was looking through the Wiki to check the Epics for each element, as I may sell off some of my monsters to save space.

FIRE:

1. Salamander
2. Burning Rogue
3. Bjarni

Honorable Mentions: Pyrotech

NATURE:

1. Ophiuchus
2. Primigenius
3. Koarim

Honorable Mentions: Cybele

EARTH:

1. Primigenius
2. Koarim
3. Avaast

Honorable Mentions: Growler

THUNDER:

1. Kaori
2. Volray
3. Bjarni

WATER:

1. Nautilus-1
2. Freeza
3. Avaast

Honorable Mention: Volray

DARK:

1. Ophiuchus
2. Growler
3. Postiguraf

Honorable Mention: Burning Rogue

MAGIC:

1. Freeza
2. Postiguraf
3. Teashire

Honorable Mention: Punchie

LIGHT:

1. Kaori
2. Daganth
3. Goathel

Honorable Mention: Wolftagnan

METAL:

1. Nautilus-1
2. Salamander
3. Pyrotech

Honorable Mention: Prototyperion

@Clackers I would would add Dragonian Beast.