Option Theif - Gradius Series: Don't know if it has an official name but this is what I call it. My father was playing Gradius III on the SNES one day and he told me about this theif that came and jacked two of his four Options. Since my dad told me about him, I had the advantage of at least knowing such a foe was in the game. You see, Options/Multiples are EVERYTHING in Gradius games. You ain't got Options in a Gradius games and you're in a tight situation, the odds of you get screwed increase ten fold. I had the rotating Options and I was in the fire level. All was going well and then I heard this noise like a siren going off. I look to the left off the screen and there it was, the Option Theif. It charged at me and by moving up, he didn't get any of my goods. :P That being said, I still consider him a threat, especially in Gradius V because I didn't see him coming one time and he jacked ALL of my Multiples!!
Medusa Heads - Castlevania Series: My freaking word, this could quite possibly be the most annoying enemy in a video game. Even when I anticipate them coming I still get knocked down steps or into a pit! Agghh!!
Quick Man - Mega Man 3: He was a push over for me in MM2 but in MM3 he's even fast and he has a flat playing field to fight you on, making him the most irsksome of all the Doc Robots for me to fight. I've even had to use E Tanks on him and I hardly ever have to resort to an E Tank on a Robot Master in a Mega Man game. He is indeed a worthy and annoying foe.