6. Mortal Kombat II - Shao Kahn
Mortal Kombat II's boss was notoriously difficult, sure he was beatable, but for most only by using repeated frezzes from Sub Zero. This was a boss designed for your destruction and your destruction alone. For the average Kombat player, there to rip the still beating heart from their enemies, Shao Kahn was a middle finger to how good you thought you were and regardless of the difficulty level you were playing at, or how easy you may have completed the run up to Kahn, the shift in difficulty was phenomenal. He would come at you with the might of Thor and pulverise you in seconds, leaving the only option a shameful press of continue so he could do it again and again and again.