I still haven't forgiven Apple for...
Well this doesn't directly answer your question about "the worst OS X version". But since you already mentioned things you haven't forgotten and haven't forgiven Apple for:
I'll never forgive Apple for what they did when they introduced iMessage. That is, they quietly started channeling SMS messages to your computer instead of your iPhone. Like WHAT THE HELL??
Suddenly, I stopped receiving any messages on my iPhone - and who the hell will look for messages on a Mac??
Took me time - and visits to my mobile service provider and the Apple store - until I realized that I also needed to disable some checkbox on BOTH my iMac and MacBook Pro to restore the ability to receive my messages on the phone.
By the time I sorted it out, I missed several important messages, which led to undesirable events (or lack of events) that were important to me back then.
So yeah, this I'll never forgive. This was also one of the main factors that made me eventually switch to Android for anything mobile.