Would that I could name only one! As much as I can name numerous specific incidents of bullying, I think that the cumulative effect of all the actions and inactions that I would describe as "bullying" in my life have been more significant than any one person's actions or inactions.
In terms of who comes to mind when I think of the word "bully," whose actions most fit the commonly understood definition of that word? That would be Jessica C., a person in my 6th grade class who I was friends with and whose later betrayal of me resulted in the only official diagnosis of PTSD I've ever received. I really didn't like her at all at first. Not for any specific reason, she just rubbed me the wrong way. I ended up feeling bad for her after a while, since she was another person who didn't really have any friends and I knew where she was coming from, and decided to give her a chance. We were never best friends, at least from my end of things, but we were the only two people that the other could hang out with at school. At some point, she learned that I was incredibly noise sensitive (as in, some sounds, especially at certain volumes, feel like someone is driving an ice pick through my ear canal and into my brain) as a result of having Asperger's and probably some separate sensory integration/processing thing, which were at that time undiagnosed and written off as me being "oversensitive." She not only started making noises around me to specifically set me off - whistles, shrieks, that kind of thing - but also told other people that I had these problems, which led to their going after me in similar ways. I was disallowed from going inside and away from other kids at lunch or break, which pretty much meant I had no escape. She eventually drove me out of the place I had hung out and eaten lunch for the entire time I was at that school for the last six years by this means and others I can't remember anymore.
The worst thing I remember was one day after school when we were the last two kids in the classroom. I was getting my violin to go to lessons down the hall. She stood in the doorway. She said something like "Do you want to play a game?" and then started chasing me through the hallways, shrieking and making every type of noise she could think of to hurt me. I finally locked myself in the empty private study room where my violin teacher was supposed to meet me and waited for her to go away. I wouldn't let my teacher in until I was sure she was alone. No one did anything about this and no one cared. There were other things that happened that year that I can't remember anymore - as in, I can remember specific conversations I had with my therapist about what was going on before or after these specific incidents without remembering the exact content of the conversations aside from being angry with her and wanting to break off with her.
It might be unfair of me to attribute the entirety of what happened to her - much of what was scary and upsetting about what happened that year was a result of other kids joining in with her in doing this and of adult authority figures who knew what was happening sitting back and doing exactly nothing. It's really the latter category of people and Jessica herself that I can't forgive. The stuff that happened that year made my sensory issues worse (contrary to adults' assertions that exposure would make me "get used to it"), since now in addition to the raw pain that certain noises caused me, I associated them with people actively trying to hurt me. What was left of my social life pretty much crashed and burned for the next three years following that, in no small part because I didn't trust other people my age and was very reactive to anything I took as a sign of betrayal.
By a somewhat more expansive definition, my parents also engaged in a significant amount of bullying. They, especially my mom, were always going back and forth between the belief that I could never, ever do certain things by virtue of who I was (E.g., my mom's insisting that I could never be a trial attorney because people would never hire me what with my facial deformity and speech impediment) and the belief that I could do and in fact control everything and anything (E.g., my dad's insistence that such parts of my medical history such as becoming severely anemic to the point of requiring blood transfusions or having a seizure in a restaurant were things I did on purpose to get back at my mom). My food limitations due to texture (again, due to Asperger's/sensory integration stuff) were always the subject of jokes or teasing, when they didn't lead to stuff like my parents forcing me to eat things that I knew would and in fact sometimes did make me sick, and in one particular instance, threatening to make me eat my vomit if I threw up. My other sensory issues and trigger-type things weren't handled much better - stuff like preventing me from leaving the theater in parts of movies that had onscreen surgery or gore, or actively preventing me from leaving situations where I was facing sensory pain or overload (including during the Jessica situation, as my mom was the teacher's assistant in my classroom that year). My father, especially leading up to and during the period of the divorce, would trap me in a car or a room with only one door, which he would be blocking, and yell at me for hours. If I argued back, I was being disrespectful and angry. If I cried, I was being manipulative. If I didn't respond at all, I was being disrespectful. Once he got going, there was no way to defuse the situation, especially if I was alone. I don't think he actually made a distinction between my mother and I, because it seemed like the things he was complaining about seemed more to do with her, whether they were true or not, than they did with me. To my mom, I was always "embarrassing" her or looking or doing something "retarded" or fucking things up by virtue of just doing what I did. She never really explained why what I did was bad, even when I asked or tried to explain my side of it, and at the same time assumed that I was acting deliberately in making things difficult for her. Both of my parents hit me - not frequently, but always for petty things, issues of control or perceived "disrespect," nothing that couldn't have been dealt with by much more gentle means. And then they both had the nerve to assume that I was the one who needed fixing, when it's how they dealt with their kid, not how I was defiant or imaginative or hypersensitive, that was wrong.