6.83 was awful. Other atrocities are the TI2 patch where Lycan was the thing (and he wasn't even the most OP hero of that patch). The week Centaur was introduced to Dota 2 with his (at the time) new Ultimate that fully stunned anyone you ran over with it, even though that was fixed with a small patch and he wasn't part of the major balance patches yet.
Earth Spirit in his introduction in 6.79 was pretty broken, and I was a filthy Earth Spirit spammer. It was basically the reverse of what it was today: Grip was the stun, and super easy to set up, and Smash was the silence.
Terrorblade when he was introduced was also broken, with a ridiculous BAT with both Meta and normal forms. Reflection was good, and of course his other skills were great too.
The TI4 patch was also a meta I didn't care for: Ram your heads into them at 5 minutes and win no matter how silly it is.