The ideal viewing order is to watch
RoboCop: The Series in the order presented on the DVDs, and then stop. R:TS is a lot of fun, albeit lighter in tone and more kid-friendly than the movies, and has a terrific cast. (
Here are my blog posts discussing/reviewing it.)
Prime Directives is dismal and stupid, and its RoboCop was completely miscast. It's not at all in continuity with R:TS, although it repurposes a bit of stock footage from the series as flashbacks, creating the misconception that they're connected.
What to keep in mind when watching R:TS is that it's a direct sequel to the original film but ignores and contradicts the theatrical sequels. This is because its pilot was rewritten from the rejected movie sequel script by the original film's writers. So it's basically an alternate reality branching off from the first movie, and in my view it's truer to the conception of
RoboCop than the movie sequels were. The supporting characters start off being just renamed versions of the movie's characters, but they end up developing their own distinct personalities, and the series' continuity and the first movie's mesh pretty well (as I discuss in more detail in my blog posts).
That said, you don't need to rewatch the original movie before R:TS. You can if you want to, but the pilot does a good job recapping the backstory.
What to keep in mind when watching
Prime Directives is, "What am I doing? They warned me not to watch this! I should really turn this off and go read a good book."