Fiend

Fiends are a particularly nasty type of card to face, with a decent mix of monsters:

• Demise, King of Armageddon
• Dark Master - Zorc
• Raviel, Lord of Phantasms
• Dark Necrofear
• Dark Ruler Ha Des
• Giant Germ
• Goblin Elite Attack Force
• Sangan
• Wall of Illusion

Fiends also power up from the Yami Field Spell Card, and since many Fiend's are Dark, also get a boost from Mystic Plasma Zone.