Here is a ranking of the top 25 best-selling Oppenheim books. This ranking is based on the number of copies of these books currently available on the used book market. The secondary book market is a good indicator of the total number of books published throughout the years.
The Great Impersonation is the most well-crafted of all the Oppenheims I have read. It is a mystery about a man who switches identity with another man. The book keeps you guessing until the last page. It is still in print and has gone through many editions through the years. It is far and away the most popular of all Oppenheim novels -- selling more than twice the number of its nearest competitor.
The Treasure House of Martin Hews is my favorite Oppenheim novel, probably because the main character in the book is a collector, like myself. It is a crime story and is not terribly sophisticated, but I do like Martin Hews.
Peter Ruff and the Double Four is a wonderful collection of stories about an unlikely detective. It is classic Victorian/Edwardian detective fiction with a clever sleuth. This is as close as Oppenheim comes to Conan Doyle or Arthur Morrison.
The Lion and the Lamb is a hard-boiled noir story about a couple of criminal low-lifes just released from prison. Great story.