I'll go with Tsunami's idea and list one for each game.
PS1 - Dark Falz. He just suddenly jumps up out of the shadows. Scary!
PS2 - Mother Brain. She's powerful enough to kill a planet and control two, so that makes her a tough nut to crack.
PS3 - Couldn't really say...
PS4 - The second Dark Force. He's got the huge tower, but it's not just a tower. It's organic looking, like the whole thing is part of his body or something. Now that's a big monster!