Probably the worst co-worker was one of my many supervisors at this big bar I worked at. She was so belittling and rude.
She would tell you off for "not holding the menu properly" or for not "standing straight enough" all while she was (illegally) selling tables, fraternizing and sleeping around with married, big-name men affiliated with the company that obviously didn't care about her.
So she would run around telling everyone about how her "new boyfriend" is such an asshole (and the more people she would complain to the more ridiculous her stories became) when really it was about 5-10 different dudes.
Not only, when the managers weren't around she would leave randomly and come back coked up as fuck (even told some of her 'friends' about actually doing it with some of these big-namers).
This woman would invite her jerk friends, give them the best tables, comp their food and liquor (shots on shots on shots) and they wouldn't even tip.
Just overall one of the most cruel, insecure, bitchy people I have ever encountered. Thankfully she was fired not long after, as she crossed way too many people and after one person complained, all her dirty laundry came to light.