The wither has 300 hearts however the ender dragon can regen health but only has 200 hearts.
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The mob with the most HP in Minecraft would be the enderdragon. The Iron Golem which is a mob you can summon or craft also has 100 HP and is very powerful killing zombies, skeletons, spiders, and slimes.
Go to minecraft wiki and do a search for "mob grinder" and it gives you a tutorial of how to do it.
You cant find a giant mob in vannilla minecraft but you can sumon him using a command or a spawner.
Mobot
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