I don't know if this one was mentioned yet, but even if it is, I still feel the need to bring it to attention.
The Genesis game Dynamite Headdy is a fun-as-hell game. A unique game from Treasure where you use your head to attack stuff. Anyways, the boss for the eight stage is without a doubt the hardest boss in the game, even harder than the final boss.
Twin Freaks. The heathen that he is. The tough part about this boss is actually a few things:
1. Auto-scrolling stage that varies speed.
2. In order to get through the stage, you have to hit a switch that flips the stage upside down. They were featured in the last stage, but they are in this stage also. However, when you flip the stage, the boss changes heads from a nice slow-walking green head to an angry fast-walking red head.
3. When you switch to the red head, the scrolling speed speeds up, he moves closer to your side of the screen (making it easier to squish you), and you can't deal damage to him.
4. You have to hit the ear of the green head in order to do damage. The red head will take no damage (besides, you'll be too worried about surviving to attack that side).
5. Spikes. Thousands of them.
6. Did I mention the scrolling speed speeds up when you go to the red head?
All of this makes for a damn hard boss fight. I've never heard of anyone beating him without losing health, and dying alot at this point is not uncommon. Maybe not the hardest boss of all time, but certainly one worth mentioning in my opinion.
