Which level 4 monster is the best?

From the castle units, Vampires will only be able to fight on equal terms with Crusaders (and in most cases, they will win). They will crush the rest. Even Golems...

Not just with Crusaders (although this is my favorite unit in the game, but we need to be realistic). Ogres can also put up a decent fight against Vampires - they have a lot of hit points, high attack, plus Bloodlust, but the rest are just easy prey.