Out of the list, Tanks. I was playing Dead Air in the hotel level. While going down one of the halls, I was peeking into each of the rooms to check for zombies and goodies. I peek into one of the rooms and find a Tank... In the doorway. Staring right at me. Then the music started. Also when one knocked me off of the roof of the Hospital in No Mercy.
Other than that, the most Hardcore enemy I've faced has to be Giygas in Earthbound, solely because of context. Four children face off against-and inside of-a swirling realm f pure chaos, the true form of a creature so powerful and evil that its attack is incomprehensible to the human mind. Attacks do nothing to it, seemingly disappearing into the shadows, and all the while the darkness is whispering broken and disturbing phrases. The only way to defeat this monstrosity is to pray, and even then the prayers are eventually swallowed by the void, relying on the player's own mental will to end it.