Principal Seymour Skinner
One of the obvious enemies. Skinner is Bart's school principal and as you should all know, Bart hates anything to do with school. In many episodes Bart has pranked Skinner including putting a burning brown bag outside his front door and putting 'kick me' on his back. Once the two became friends after Bart had got Skinner fired but when Bart got Skinner his job back they became enemies again.
The biggest and most deadly enemy of all! Bart has foiled Bob on many occasions, causing Bob to want to kill Bart. Every time Bob has tried to murder him, Bart has foiled him again and again making Bob more angrier and angrier every time.
When Snake got caught smoking he was sentenced to the chair. Bart was a witness and Snake warned that he would kill Bart and the other 2 witnesses. Homer later got a hair transplant that came from Snake. Homer developed Snake's habits and killed 2 witnesses. When Bart was locked in his room with Homer, he nearly got murdered but was saved when the family and the police broke into the room.
Willie isn't a recognized enemy of Bart's but he swore to kill him when Bart filled his cabin with food. Willie also tried killing Bart, along with every other child in their dreams when he was set on fire and died.
There are also many more enemies that Bart has got over the years including Lester and the school bullies, Jimbo, Nelson and Kearny.