Mobzilla (based on Toho's Godzilla) is the strongest hostile mob in Minecraft, added in the 50.6 update. It only spawns in Ocean and Plains biomes and is not manually summoned. Its arrival is always marked with a massive storm 10x more dangerous than typical ones. Also, for good measure, around 50 Iron Armor Zombies are following him at all times, fending off any players.
Stats
Hearts: 2000
Damage Value: 10 Hearts per kick.
Armor: 40
Abilities
Kick/Punch - Standard melee attack, temporarily displaces target 2 blocks in the air.
Atomic Breath - Launches a barrage of dozens of Ghast and Blaze bombs.
Lightning - All players within the immediate vicinity of Mobzilla will periodically be hit with lightning bolts.
Seismic Attack - Tosses all nearby players 10 blocks into the air.
Lava Breath - Enough said.
Roar - Gives all nearby players a Weakness and Slowness status effect for 20 seconds.