I think The Original Series was the most appealing and satisfying Star Trek for both the casual and also the most determined, invested viewer until at least the 2009 movie came along and, arguably, seduced the casual audience yet more successfully.
Now is a good time to add them to yours, if you have't already. Amazon is running their Black Friday Deals of the week and today's Deal of the Day all things Star Trek. The Star Trek: The Complete Original Series (Seasons 1-3) [Blu-ray] box’s SRP is
All of the discs included are region free, so they’ll work internationally, and there’s plenty of supplements including alternate versions of episodes and tributes to the talent. Above all else, though, there’s the astonishing Blu-ray presentation. These episodes never looked this good on broadcast, and I found some scenes to be absolutely jaw-dropping.