Favorite: Small Slime. They try to attack you but can't so they just look derpy.
Least Favorite: Small Magma Cubes. They look tiny like small slimes but they attack you!
Hardest: Wither. Wither by FAR.
Nerfed: None of them. If you use the right tactics (Sprint-hits for creepers, bows for ghasts and skeles, fire resistance pots/armor for ghasts, friends or highly enchanted stuff and good tactics for wither, and on and on you can beat them easily.)
Favorite:Wither Skeleton-just epic
Least Favorite:Slime-There weak,at least magma cubes have 48% damage reduce.
Hardest:Zombie Pigman-Just because they attack in groups
Nerfred:Zombie and Zombie Villager(1.6)-Made them TO hard.
Upgrade:Ghast-everytime the game tries to spawn a Ghast,95% of the time it fails,make them spawn more often!Easiest:Small slime and magma cube-half a heart?really?