3. Rodin - Bayonetta
If players are committed enough to collect 10,000,000 Halos on a single Bayonetta save file, they'll be given the opportunity to buy the Platinum Ticket at Rodin's bar the Gates of Hell for 999,999 Halos.
At this point, you'll be transported to a celestial arena for a secret boss fight against Rodin's true angelic form, who is by far the most difficult boss in the entire game.
The fight is automatically set to the highest gameplay difficulty, where he's immune to being stunned, can shave off giant chunks of your health in a few quick hits, and has a diverse platter of attacks to blindside you with.
You need to keep moving, but even then it's easy to make a simple mistake which tips the fight irrevocably in Rodin's favour. If you do manage to win the fight, you'll be gifted with the enormously powerful, eponymously named Rodin weapon.
You also get to fight Rodin again in similar circumstances in Bayonetta 2, though perhaps as a result of the fan outcry it's not quite this punishing.