I still stand by CBB18 was just a month-long sprawl of a charmless, vapid thug committing psychological torment full of the highest passive-aggression mixed with full-on aggression towards almost everyone he lived with. Technically bullying, but several times actual abuse. Worst person I've ever seen on the show.
Along with this, I'd still agree with the mention of Shahbaz, Isabelle, Jale and Jodie that others have pointed out (I don't think Deana was by a lot of people though, I'd really only consider bullying of her to come from Conor and to a lesser extent Arron, while the rest of the insiders were just rude to her really). I would also point out in BB15 when Helen took a very minor joke as a personal attack and followed Matthew into the garden to get in his face was the most physically uncomfortable I felt watching a scene from the civilian version (in addition to the time she outed Ashleigh as a backstabber in the garden in front of everyone and forced an argument out of her and then afterwards Helen said "I couldn't have been bullying her because I had a whole crowd of people there to speak up if I took it too far"
).
I also think Gerry very much bullied Charley and especially Nicky in BB8 by goading them into arguments in bringing up personal matters all the time at them and generally rising things to an aggressive point with them when there wasn't much need to be, which would often lead to THEM being villainised for how they would react to HIS actions. Another easy bottom 5 housemate choice of mine.
Outside of that, there's many times individuals were mistreated but I couldn't consider it enough to call it bullying. This is including but not limited to, Alexandra DeGale was often cited as one of the biggest bullies to ever be on the show, but really she was just a rude person defensive about everything, who I did dislike but I didn't think she was really as bad as the other people I mentioned.