Which creatures would you like most in Minecraft as a standard mob (so there are no mods in Minecraft)?
Dragon is more or less.
Although Aquatic is already and good, even more sea animals are always beautiful. Then the water is sometimes dangerous.
Dragon as a standard mob? Hmm, destructive and nice idea, but they should occur less often.
Vampire. Would be an interesting opponent spawns at night or in caves. Sometimes he lurks you as a bat.