According to Elizabeth Breck
In October 2021, Elizabeth Breck, licensed private investigator and the author of the Madison Kelly private eye series,  wrote a short piece for CrimeReads  on the five fictional sleuths (and one special mention) who she thought did the job right.
The choices surprised me at first, but she makes a solid case for each and every one of them.
- Jane Whitefield by Thomas Perry
“… the best PI in fiction. And technically she isn’t even a PI.” - Kinsey Millhone by Sue Grafton
“I don’t know how Sue Grafton did it, but Kinsey’s investigation skills were always spot-on.” - Nero Wolfe by Rex Stout
“Rex Stout writes it exactly how it would be done; it’s like a training manual.” - Stephanie Plum by Janet Evanovich
“Sure, there is hilarity and downright schtick, but even a real-life PI can suspend some disbelief to have a good laugh. Janet gets it right on the serious stuff, where it matters.” - Elvis Cole by Robert Crais
“Everything Robert has Elvis and Joe do is realistic, especially with tailing subjects and interviewing witnesses.”
And one more special mention…
- Harriet the Spy by Louise Fitzhugh
“I couldn’t make this list without mentioning the one who started it all for me… It seems Ms. Fitzhugh got a lot more than just investigations right in this book.
Intrigued? Don’t take my word for it. Head on over to The Five Most Realistic Pis In Fiction, and read the full article.