Passive Mob: Probably either Sheep or Pig, they are adorable Hostile Mob: Spiders and Endermen. They can be considered Neutral at times and then Hostile. Spiders are actually Neutral during the day, as some of you may know, and everybody knows what happens to Endermen when you look them in the eye T.T
It's pretty awesome, leaves an egg you can collect, and its own death provides a lot of experience points. It even provides a portal back to the overworld, what a nice dragon. Vanquishing it even provides a humongous amount of words that are essentially marking the "end" of minecraft.
Sheeeepie sheeepie sheeepie. Wool is a great building material.
At the moment, I am especially hostile to creepers, because they blew the crap out of my railway switch/dynamic lighting redstone circuits, which is weird because there is no way in or out of it and it happened when I wasn't near it.. I opened my back door and boom.
My favorite mob would have to be the pig. Ever since I started minecraft I always like the derpy looking pig. I also take advantage of the "saddle" and "carrot on a stick" making it the only mob to be ridden and controlled. (Until horses come out of course)