What is the most ridiculous balance decision ever made in the game that you can think of ?
For me ,it would have to be sky rocketing 
's AP rations. Getting deleted by Q was fun
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Hiring certainT
Beat me to it
Cho Gath Oneshot with Gargoyles was NUTS.
I dont think thats still around thankfully.
Patch 7.4:
Feast REMOVED: Loses half stacks on death. NEW EFFECT: Can now stack infinitely (from 6), but can only gain up to 5 stacks from killing non-epic monsters or minions. REMOVED: Healing if at 6 stacks. REMOVED: Champion kill granting 2 stacks. REMOVED: Minions and monsters kill refunding 50% cooldown and mana cost. NEW EFFECT: Tooltip now lists how many minions, pets, monsters, dragons, epic monsters and champions you have eaten. AP ratio reduced to 50% AP from 70% AP. NEW EFFECT: Damage now scales with 10% bonus health. Health per stack reduced to 80 / 110 / 140 from 100 / 140 / 180. NEW EFFECT: Reaches maximum size at 10 stacks. Maximum bonus attack range increased to 75 from 50.
Nasus R changes with half second Q cooldown is a close second.
The assassin nerfs while massively buffing supports and adc
My top 3 are:
Removal of Tracker's Knife for 99% of league because the top 1% was building (and abusing) it.
Feral Flare being allowed on live servers in the state it was in on release (infinitely stacking on-hit damage and healing for 1650 gold and a few camps after buying Wriggle's Lantern, along with base stats that worth quite a bit more than the gold cost from Wriggle's Lantern at the time)
Guinsoo's Rageblade being turned into a Tiamat effect that stacked with an Actual Tiamat without a delay on multiple targets in early season 6, which ultimately led to the removal of Devourer.
The Juggernaut patch.
70% winrate
mandatory pick or ban
hotfixed into a monster
All right before worlds.
Oh, and 
existed.
Does Dynamic Queue count?
When they started to do the class reworks instead of addressing individual problem champions.
Buffing ADCs on the ADCs rework and reducing 
prices.
Sivir extra 60 damage on her Q at lvl 1 . It did stay 1 hours after they hotfix and they said that they went overboard on this change lawl .
There was about a week on the PBE where Jinx's Zap! could critically strike.
Buffing 
when she was already fine.
For me, either the massive you mentioned, making 
a walking Deathcap, or whenever they Sejuani, Ezreal, etc. a champion by nerfing them and not what's making them op (or vice versa).
And class updates. We got some good things out of them that people liked, but for the most part they were failures that broke champions and their items.
% Health.Fucking.True.Damage.
Giving assassins who are already overpowered some "compensation" for nerfing duskblade, completely defeating the purpose of the item nerf.
Almost all of patch 5.22. That patch has ruined the game for years.
Release Xin and release LB. Xin was the most broken champ in league history on release, and LB literally invented hot fixes for Riot.
They removed revive for no reason{{summoner:36}}
I want to say something about Ornn
.
I loved him at release, but now he just doesn't feel the same.
[cass-cry]
I'm going to have to just say the entirety of Season 2. Because Draven was allowed to be the most powerful champion in the game for the entirety. If you just stacked Bloodthirsters on Draven back then you could one shot any adc who didn't stop building damage. You could one shot most supports. You could two shot any mid laner you couldn't one shot. You could out duel any dueling champion. And you could out tank any tank because of how much lifesteal you had. And on top of all of that the game itself had less damage so Draven being able to literally one shot most champions while simultaneously having the most sustain, cause you'd heal 100% for each auto actually made you unstoppable. I remember one time I was playing with my friends and jokingly said I was going to use "afk strats" clicked on a tower and took my hands off my keboard and mouse. I destroyed the tower and got a pentakill with only that single input because Draven was just that OP.
messing with galio, a tanks, ap ratios that made his one shots nutty. the fact this made it through is insane
Soraka rework.
I'll admit my bae needed a rework, but what they handed us.... wasn't it.
rework, and making him unable to use his W without an ally around, DESPITE being a solo laner before the changes. Scary juggernaut that needs someone to hold his hand.
Besides the bias and the one about Galio you already mentioned ~
I think making Q crit was one of the craziest changes. That was the patch where she would tumble towards you and one shot you because of a crit, it was not fun at all
Old Yi had a 1.0 AP Ratio on his Q, and a 4.0 AP ration on his W. New players literally have no idea.
there are over 1000 (literally) bad balance decisions that i can't pick only one.
ardent censor being straight cancer for 6 straight months when every human being knew it needed to be gutted.
6 straight months of that cancr because "muh lcs" and "muh balance team needs time to figure out how to not be retarded"
Zoe
when Yorick was finally a viable toplaner who was actually pretty good at his splitpushing, a patch or two before preseason, and was instantly swatted back down because apparently unless you like Illaoi or darius (i love them both, but still), you're not allowed to player juggernauts ever until they pop back into viability, and then everyone whines because characters with barely any mobility besides maybe a speed boost or CC/gapcloser to get close kill people they're close to: newsflash: If they're close enough to kill you THEY'RE CLOSE ENOUGH TO KILL YOU.
Edit: These I just made up for fun. Apologies!
Nasus: Siphoning Strike Grants 1 Bonus AD instead of 3 damage on Siphoning Strike.
Quinn: Not affected by movement speed soft or hard cap.
Zoe: Speed of spell projectiles scales with AP.
Cho’Gath: Can cast Rupture under self, unstoppably flying in the direction of movement if walking or displaced when hit by Rupture.
Blitzcrank: Hook resets on champion hit.