The hardest Genesis/Mega Drive games?

What are considered the hardest games on Genesis/Mega Drive?

For example, the NES has a few famous ones like Battletoads, Silver Surfer, Dr Jekyl and Mr Hyde, Castlevania 3, Ninja Gaiden etc.

I've never really heard anyone discuss the toughest games for Genesis/Mega Drive and was wondering what people think?

That I've personally played it's Genesis Battletoads, The Immortal and Last Battle, all of those games are tough and I've never beaten them. Zombies Ate My Neighbors is also fairly tough although you get passwords. I also found Gods to be pretty hard, and The Pagemaster is pretty tough for a platformer.

What's yours?