Standalone or Series?

As readers, do you prefer standalone or series? As a writer, I have written both. My first publication was the six book Paul Jacobson Geezer-lit Mystery Series. I also have several books with sequels: The Back Wing and The Front Wing; Court Trouble and Paradise Court. I also have the Omnipodge Trilogy of Old Detectives Home, Last Gasp Motel, and A Mystery Yarn. My other nine published books are standalones. The reason for standalones are that I get an idea and want to pursue it even though it might not lead to a series. Some examples: Unstuff Your Stuff is a mystery that came about because I got interested in what professional organizers do. Murder in the Museo Kircheriano is a historical mystery based on the fascinating character Athanasius Kircher who was know as the last man to know everything. The Tesla Legacy is a thriller based upon inventions of the brilliant inventor Nikola Tesla. Mystery of the Dinner Playhouse came about because of family members who have been in the theater world. Murder on the Switzerland Trail is a historical mystery set in the mountains of Colorado where I hiked on all the publicly available sections of what had once been a narrow-gauge railroad bed. The Best Chicken Thief is a biography of a World War II veteran I met who was in the infantry in Europe, captured by the Germans and liberated by the Russians. The VV Agency is a paranormal private investigator novel and happened because my then agent suggested that I should explore a paranormal mystery. The idea for Death of a Scam Artist came about when I became intrigued with elder abuse and people who preyed on seniors. My upcoming middle grade mystery, The Adventures of Sure Luck Holmes, which will be released in November can become a series.