The wither is one of the hardest to defeat mobs in Minecraft, you can start by shooting arrows, then when the wither starts to fall down and turn blue, that is the time to take out your sword! A fun way to defeat the wither is with bees, spawn a lot of bees(at least 30) and sit back and relax!
2#: Zombie pigman:
Zombie pigman is not that strong, but thousands of them attacking you will be just irritating, they will not attack you if you do not attack them, if you do, my best advice will be to use your shield and move back, try to make them fall into lava, do not use fire, that will do nothing, rather just don’t mess with them.
I don’t know why I put them 3rd, I spawned 64 of them and went into survival mode as a challenge, I equipped myself with full gear, and, lost. They are strong, I will advise you to invest in a shield. They can’t get through the shield.