I'm inclined to say the watcher knights from hollow knight, but this little voice is telling me something different.
Miiiinecraaaft, it whispers.
Well then.
I'd say it's a tie between the enderdragon and the wither. The enderdragon requires you to get 12 ender pearls(an uncommon drop from an uncommon enemy) to access, and you need to make them into eyes of ender and throw them into the air so they float in the direction of a stronghold so you can follow them, but they have a chance to break when you use them, and once you're above the stronghold(they're underground) they just go down into the ground and you don't have much hope of getting them back. So when you finally find the stronghold, you need to activate the portal with up to 12 eyes of ender, and then when you go through the portal you can't go back until you either die or kill the dragon, and sometimes you can spawn on a platform above the void(kills you and deletes your items) a good ways from the main floating island, which gives the dragon plenty of opportunity to knock you off(in all senses of the term), and once you get to the island you have to avoid looking at the 4890279 endermen there because they'll attack you if you do, and the dragon vaporizes any block you place on contact, and it constantly heals until you destroy a bunch of ender crystals which are situated atop really high towers, and if you pillar up to try to break them up close they explode and kill you, and finally the frickin dragon just flies around ignoring you for like an hour and you have to slowly whittle it's health down with arrows, and if you run out you're out of luck because sword enchantments deal no damage to the dragon, and then when you finally kill it it just drops a bunch of xp and an egg which is difficult to collect and sometimes falls into the exit portal and vaporizes, and when you leave you go back to your spawn point so if you want to go back to the end(the place the first portal takes you), you have to go all the way back to the stronghold.
AND THEN THERE'S THE WITHER!
So the frickn wither requires 3 wither skeleton skulls to spawn, which are very rare drops from rare enemies, and then when you spawn it it explodes after a few seconds, and just hovers directly over you so all you see is its butt(or spine, as it were) and constantly shoots exploding poison skulls at you, and it can break almost any block including obsidian(nigh indestructible), and it constantly heals, and if you die it'll most likely blow up your stuff, and even if it doesn't you have to get past it to retrieve your stuff before 5 minutes are up or it vanishes forever, and when you do kill it the whole thing is over in about a minute or less with either you or it dying, and after all that trouble, it drops a FRICKIN LAMP!
Ok it's a beacon, but it just shoots a glowy beam into the air and gives you a status effect like speed or regeneration if you put it on top of a pyramid of like 382561789 ore blocks which are made with 9 pieces of certain ores so that's like 8359187589139 pieces and you get a FRICKN LAMP.
Mojang are not the best game developers.