The original Star Trek television series ran for three years starting in 1966 -- and the first season was the best, according to IMDB. With an overall rating of 8.4, this is the second highest rated series, but it features the highest rated individual episode: Season one's "The City on the Edge of Forever," which pulled in a 9.3 rating. (Surprisingly, "The Trouble with Tribbles" only pulled a 8.9.)
The lowest rated episode was season three's "And the Children Shall Lead" -- about a colony run by children after all the adults die.
Star Trek: The Next Generation, the 1987 follow up that has spawned seven seasons and so many of the Internet's favorite gifs and memes, is also the highest rated series overall on IMDB at 8.7. While no episodes matched the 9.3 of "The City on the Edge of Forever," three episodes are rated at 9.1: Both episodes in the two part "The Best of Both Words" Borg-centered story arc, and "The Inner Light" -- an Hugo Award winning episode where Captain Jean-Luc Picard lives an entire lifetime on an alien planet as a delusional man who dreams he is a star ship captain.
But TNG also features the worst rated episode of all the series. The season two episode "Shades of Gray" garnered only a 3.3. The episode revolves around first officer William Riker fending off an infection of his nervous system. Other poorly rated episode included the the season seven Doctor Beverly Crusher centered episode "Sub Rosa" and a season one episode about Lieutenant Tasha Yar, which earned 5.1 and 5.2 ratings respectively.
But despite the lower overall rating, DS9 was a pretty consistent performer.The lowest rated episode, season five's "Let He Who Is Without Sin..." is rated 5.4.
Like DS9, Voyager was fairly consistent, but also fairly consistently slightly below previous series in terms of IMDB user ratings. The lowest rated episode was season two's Season 2 episode "Threshold."
But the final episode, "These Are the Voyages..." was its lowest at 5.5. In the episode, TNG's Riker uses the holodeck to relive the mission of the first Enterprise. Considering Riker's central role in the lowest rated episode across all of the series, the data just might be telling us that fans don't care for actor Jonathan Frakes.