The Original Series is technicolor sci-fi horror dressed as episodic space exploration. It's campy and melodramatic and by god does it know it. Some episodes were blatantly "what's available on the back-lot next month?" and are a coin toss between stupid and incredible.

TOS had some movies (roman numerals) and they range from "what if an episode was three hours long" to "modern-day San Francisco zoo heist." At their best they're fun and at their worst they're even more fun.

The Next Generation is high-concept ethical debate framed as the experiences of naval officers. The cast is seriously talented and the writing is usually excellent.

TNG had some movies (bald guy on the poster) and they were written by people who didn't like the show for people who didn't watch the show. You have to turn your brain off, but they're well-directed.