The Meaning of Life, the Universe, and Everything.
Before 1.8, it was the ghast for me. Now it's EVERYTHING SINCE 1.8. Silverfish randomly appearing in large groups because i dug a stone brick, horrifyingly powerful enderdragons, endermen(ironically), cave spiders with a poison touch, and blazes that are actually not much worse than the ghast now that I think about it. Minecraft has become a much more dangerous place.
You go looking for their delcious beef and you notice their strange purple eyes.
You think it is a glitch in the texture so you continue towards the cow.
You hit it and it is gone! You think you have killed it and got no drops.
Then at night when you are down in a abandoned mine and you least expect it BAM there it is coming at you.
You run as fast as you can but your legs can't keep up with teleportation.
You die with the taste of blood in your mouth.
Cave Spiders hands down! Thier attacks dont do much damage, however they have poison. Sure, It can't kill you. But, It will get your health really low. As for the milk bucket antidote, Very few people actually carry one down mining with them. D=
Once you get pretty far into the game, no monster is even worth looking at. Even the Enderdragon becomes an archery target practice.
However I do know what mobs I do hate...
SKELETONS - They do basically no damage once you reach the level of enchanted diamond armor, but the fact that they can ALWAYS get a shot at you and knock you back. Ugh, just annoying.
CAVE SPIDERS - They are the definition of hell. I was making an XP farm with 3 cave spider spawners from an Abandoned Mineshaft on an SMP server. Oh lord. It was 4 hours of torture... I was attempting to light everything up while 6 of these things would hit me from like 100 feet away and poison me. The moment I realized that milk would do the trick I remembered that it takes 10 years for Steve to eat or drink something. By the time I'm done with the milk, I'm poisoned again. Woot. Perfect. I died 24 times. Within those 4 hours. I'm not even kidding... Pissed me off. Reason I don't play SMP anymore. :l
Enraged Zombie Pigmen hands down. Once you annoy one of them its just run away and don't look back. I once spent 30 minutes trying to box in a zombie pigman that was after me. The thing is that once you hit them, all zombie pigmen in the vicinity attack you, so when you're running away and you accidentally hit one, its game over.
I hate the sound magma cubes make, it makes me shudder... though theyre not scary when you see them. I'd have to say Endermen, their sounds are sorta freaky and whenever they teleport away I jump at every sound. plus, how they open their mouth when the are angry.
Ghasts. Those freaking baby faced freaks always sound like their right behind you, breathing down your neck. Not to mention the fact that that make those horrid sounds when they fire their fireballs. I've lost many diamonds due to these ghost like squid...things. Oh yeah, i hate how their hit boxs are off so hitting them in the face doesn't do crap.
No mobs are scary, they just want to know you. For example, the creepers wants a hug. Spiders are lonely and wants a kiss. Zombies just want to dance with you. Skeletons are suspicious of you, so when you get close enough, YOU scare the **** out of them, that is why they shoot. Endermens have no friends, so they steal your blocks, so when you see him, he thinks "Oh ****!, he have seen me! Okay, just stare back..... can't take this anymore!!! He must die!!!
And animals are just stupid.
So there you have it, none of them are scary. But players, players are scary!