Most Difficult or Hardest Enemy or Boss you have ever Encountered?

ninja gaiden 1 - alma

FF XII - last boss

Armored Core: For Answer, Final mission (Arteria Carpals) on hard.

Dont remember the enemies(4) names in the mission.

Megaman 2 Dr. Wily in his various forms.

Not to mention..having to re-kill all the robots. Ugh.