Which cop show was the original and what's the most popular cop show of all time?

Dragnet is the first one I remember. Keep in mind that most people didn't have tvs until the mid to late 50s. I don't know how you would determine the "most popular". Do you mean the one that stayed on tv the longest? Comparing ratings wouldn't be very accurate- many people didn't have tvs in the early days, then there were the days with only the 3 networks to watch, and now there are hundreds of choices. Also, the population has grown a lot since the 50s so there are just more people to watch everything. Murder, She Wrote was certainly on for a long time, but Law & Order SVU probably has the record for longest on air.

Sherlock Holmes, the books. Then Agatha Christie. TV shows and films copied them.

Dragnet would probably be considered the pioneer.