The battle of Assassins and Tanks
Tank is the opposite of assassin, assassin's job is to kill the most valued target, as fast as possible, while tank's job is to suck damages from the most valued target, as much as possible. if X team has no tank but has one assassin vs Y team which has no assassin but has one tank, Y assassin will jump in the X ADC and mage, while X tank will rush to Y ADC and mage, peel, tanking, and zone them with its body.
in the theory, this should create balance, but in the reality
assassin considered as OP by community

Assassin rarely talk about

Tank considered as OP by community

Tank rarely talk about

"but muh assassin/tank not on the list" they are somewhere in between,
so why the hell?? Tanks and assassins are in the same way, but in different direction. So the only possible answer is the power differences between tanks and assassins. Let me put an example of this
Assassin fed, "GG they even melt tanks" Tank fed, "Don't focus tank"
Countering assassin "learn to CC" Countering tank "kite, CC, items that melt tanks"
Laning vs assassin "don't get them fed, ever" Laning vs tank "dun worry, let's bully them with ranged"
Assassin get penta "GG gj nc let's push win" Tank tanking 5 man "lol enemy team bad, focusing tank"
but why
are considered as OP ??
well that's because they offer a lot for their team, and have less counterplay, ali can bring squishy to their team, great diving ability, rammus can "silence" enemy mage and ADC, has insane ms boost to fight kiting, and maokai literally suck damage
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) Tanks with targeted gapclosers
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) Assassins with targeted gapclosers
Champions with targeted gapclosers that are played less often