It's interesting to compare Jane's trajectory with other long-running detective characters. Rebus or Morse have longevity, but their core selves are pretty fixed. Jane is arguably one of the most dynamic, changing protagonists in the genre. That's why the sequence of the books is their most important feature.
Wait, are we talking about the Lynda La Plante books? The Prime Suspect prequels? I think I saw the TV show but never read them. Are the books way better? Might have to check my library app.
The beauty of this question is the ambiguity. Is it her first case as a detective in charge? That's 'Prime Suspect'. Is it the first major murder she works on? That's likely in 'Tennison'. The prequel novels flesh out her early career wonderfully. In 'Tennison', the 'Murder Mile' case is huge, sprawling, and deeply personal for the community. She's not leading it, but she's deeply involved in a way that changes her. Both books are worth reading to see the full arc from rookie to seasoned pro.