Technically, it is the giant, because it cannot be spawned in creative or survival. You require a 3rd party program to spawn the giant. The rarest mob that is spawned, is the enderdragon. There is only 1 in The End.
Go to minecraft wiki and do a search for "mob grinder" and it gives you a tutorial of how to do it.
Mobot
witherboss
You can get a mob spawner in Minecraft by using any pickaxe (except wood) with Silk Touch 1 enchantment or by spawning it in (Dec:52).