The first ever TV series of Star Trek, featuring the most popular science fiction characters of all time, such as Captain Kirk and Spock.

You might be put off by some things. The planet backdrops and creatures' faces/masks are made on a cheap budget. The special effects and sound effects might not be as good as some new age sci fi fans are used to.
And the composed soundtracks of that time weren't as good at making atmosphere and emotion come to life effectively. Also, the acting is a little cheesier than later versions.

But it is a classic!

The stories are intriguing and thought provoking- for the time the series was first released and now. And the array of colorful characters and vastness of different cultures and explorations is delightful.

This is what all future Star Trek series were based on! A Star Trek fan has to watch it at least once to appreciate it. I still think this is the best of all the series.