I know the Metal Gear games are KNOWN for their awesome boss fights, but you need to understand, this was THE FIGHT that convinced everyone of that. Though every boss fight from Metal Gear Solid is awesome, there are few boss fights in history that had the impact that the Psycho Mantis fight had on me, and on MANY gamers. The character design was amazing. The voice over work for the character is unnerving and befitting, and then the fourth wall shattering that happens is just, well, in my honest opinion, one of the most progressive and brilliant moments in gaming, ever. Yes. Ever.
The fact that this boss was making fun of me because I saved my games way too much, to the fact that he approved of my Castlevania fanboy status meant that the Psycho Mantis fight did one very simple thing, and still no one has been able to touch it in sense of the impact it had. He read your memory card.
Yup. That was the kicker. He would “read” your memory card, and insult you a few times accordingly based on how you played and what you saved. Add to that the fact that he manipulated your actual controller, making you need to switch ports, and you have a boss fight that will always be one of the greatest of all time.