Despite its seven-year run, Star Trek: Voyager failed to create any classic Star Trek villains to act as the series' main antagonists. Star Trek: Voyager, which ran from 1995 to 2001, told the story of the crew of the USS Voyager under the command of Captain Kathryn Janeway. During the pilot episode, Voyager and her crew were stranded in the Delta Quadrant by an entity known as the Caretaker and worked for the rest of the seven seasons to find a way home, traveling through a previously uncharted region of the galaxy along the way.
Because of Voyager's premise, the series had the job of creating a whole host of new and interesting alien species, especially ones that could serve as villains for the new show. This was different from the previous Star Trek shows which had benefitted from being set in a region of space with well-established villains, such as the Klingons, Romulans, and Cardassians. Instead of these, Star Trek: Voyager featured antagonists such as the Kazon and the Vidiians in the first season, and later the Hirogen as the series progressed. While there were a number of smaller one-off villains, these three species were the antagonists that had the most recurring storylines during Voyager's run.
Unfortunately for the show, their attempts to create a villain that the audience liked resulted almost universally in failure. While the Vidiians were compelling and provided the series with several great episodes, Voyager's ultimate goal of getting back to the Alpha Quadrant meant that they moved out of Vidiian space rather quickly and the show was forced to leave them behind. The Hirogen were similarly interesting in some aspects, but their warrior culture and obsession with "the hunt" made them perhaps a little too derivative of such classic villains as the Klingons and Cardassians. The Kazon fell into a similar trap as the Vidiians, not making it out of the 2nd season because of Voyager leaving their space. However, they also had the disadvantage of being unanimously hated by fans, many of whom thought their appearance was ridiculous and most of their storylines were boring.
The producers on Star Trek: Voyager seemed to realize in later seasons that they had failed to create a good villain, which resulted in them bringing the Borg, a truly classic Star Trek antagonist, for the latter half of the show. The Borg made their first appearance in season 4, with the addition of Seven of Nine, a former Borg drone, to the crew. Because Star Trek canon had stated before that the Delta Quadrant was the place where the Borg originated, they were the only established Star Trek villain that the Voyager crew could realistically encounter. Seven's inclusion on the show lead to many storylines involving the Borg during Voyager's final four seasons, and they even ended up being integral to the series finale, unwittingly helping the crew finally get home.
The Borg's inclusion ultimately saved Voyager's villain problem. Indeed, one of the things the show is best remembered for is its expansion of the Borg mythos. It is unfortunate, however, that Star Trek: Voyager never had any success in creating any classic Star Trek villains to add to the canon. While some of the show's antagonists showed promise and provided a few good storylines, it was ultimately forced to fall back on a well-established enemy to keep fans coming back for more.