Derrick’s not even close. It used to be an export hit but was fairly grounded in 70s/80s schematic telly fodder. While half of Europe has watched Derrick at a time today’s audiences would struggle to see its appeal. It still has its fans of course. But its cultural impact seems at best questionable today - like most tv shows of that era. It ran for 24 years and a bit less than 300 episodes. But there is hardly a significant difference between the first and last one.
Tatort (scene-of-crime) on the other hand is closing in on its 50th anniversary. Over the decades the show has acted as a cultural yardstick not only for tv entertainment and workmanship in film production, it’s also been a social and political indicator with its themes and characters, its storylines never far from the pulse of the Bundesrepublik. With its anthology format - housing a variety of investigator teams - it has tackled every facet of the genre from Whodunnit to thriller to satire, and everything in between.
In short, you may watch the entire Derrick canon and still not get more acquainted with German reality and its customs. But if you just watch three different Tatort episodes from each decade you will get a pretty good idea which things concerned Germans during the respective times and how they were wrapped into tv entertainment.