Angus565 said:
Spiders for sure. They're about the only thing that can get over my 4 block high wall surrounding my house. They have caused me a lot of grief in Multiplayer.
Spiders can climb up walls, you need a block jutting out at the top to stop them climbing over (they can climb directly up a smooth face, but any overhang will block them)
I don't find any of the monsters all that easy. Zombies seem to do a lot of damage very quickly when they get up close, spiders jump at you and normally manage a couple of hits before you kill them, skeletons have ranged weapons and creepers blow everything the fuck up.
Creepers are however the only ones that can approach silently and kill you before you know what's happening, so I guess they're the most deadly.