In The Transformers, there is no greater thorn in Megatron's side than Starscream. As aerial commander and Megatron's second-in-command, Starscream has unprecedented knowledge and access to many of the Decepticons' plans. However, he always tries to use his status to usurp or even kill Megatron and take over as leader. While his constant attempts fail, one character in Beast Wars embodies all of his qualities and uses them to far greater results.
In Beast Wars, Blackarachnia is created by the Predacon Tarantulas to serve within their ranks. While she is mostly loyal to the cause, she has no real stake in them. As a result, working for her own best interests is far easier than if Starscream did. By not having any true allegiance to either side, she is able to exploit the Predacon's soldiers and even double-cross them from time to time. Like Starscream, she even butts heads with her continuity's Megatron.
What makes Starscream such a liability in terms of others joining his side is that he is overly cocky and vocal about his secret plans. This often leads to most of the Decepticons being very aware of his attempts at a takeover but thinking nothing of it because he always fails. Even Megatron knows of Starscream's backstabbing tendencies. However, Megatron is immensely stronger than him and knows that he is too great of an Air Commander to kill when they're in the midst of a war. Even though Megatron lets him live, that doesn't mean he doesn't punish Starscream for his failed attempts at usurping his rule.
Blackarachnia, on the other hand, only desires independence and power and will work with either side to get it. This same hunger also gives her a Transmetal upgrade, but before she gets it, she seemingly dies while working with the Maximals to clear her Predacon programming. As a result, she later earns her Transmetal form while also quietly revealing the real reason she is a much better version of Starscream.
When Blackarachnia was created, her base was of a Maximal protoform. This meant that there was still goodness in her spark that was otherwise overshadowed by Predacon programming. The reason she is so good at playing both sides is that her spark conflicts with its true self and its programming. By "dying," she freed herself of that and gained the independence she always wanted. It also proves that she is a survivor, like Starscream.
While both characters will do anything they can to survive, the problem with Starscream is that he isn't fighting for any better cause than to be better than Megatron. While he is adept at running away, he can't escape his former leader, who returns only to kill him after getting a new upgrade. Blackarachnia, however, fights for herself and her own wants. She doesn't want to lead or kill her leader. Instead, she wants to make her own choices and live freely. So, while Blackarachnia is similar to Starscream, her survivability and desire to do what's best for herself are ultimately what makes her the best.