First things first, you can't really answer this in general because each book in J.K. Rowling's 'Cormoran Strike' series (written under the Robert Galbraith pen name) has its own cast of suspects. It's not like a TV show with a recurring villain. Every mystery is self-contained. So the main suspects are always different, and that's part of the charm—you get a whole new web of lies and motives each time.
Take 'The Cuckoo's Calling' for instance. The death of Lula Landry seems like a suicide, but Strike is hired to prove otherwise. The main suspects end up being people from her glamorous but toxic inner circle: her adoptive brother, her biological brother who showed up out of nowhere, her weirdly obsessed designer, and her rock star boyfriend. The book does a great job of making you suspect every single one of them at different points.
Then in 'The Silkworm', you've got the bizarre world of publishing and a missing author. Suspects include his estranged wife, his jealous fellow writers, a rival editor, and his own literary agent. The pool is smaller but way more vicious, all competing in this petty, backstabbing industry. By 'Career of Evil', the dynamic shifts completely—it's a serial killer targeting Strike through Robin, so the suspects are three men from Strike's past he personally believes could be capable of such brutality. It's much more personal and grimy than the high-society crime of the first book.
So the 'who' totally depends on the 'which book.' If you're looking for a common thread, it's that Galbraith excels at creating ensembles where everyone has a secret, and the true culprit often feels both shocking and inevitable once all the pieces come together. The journey with Strike and Robin through that muddle is what makes the books so addictive.