As one of the longest-running and most successful science fiction franchises of all time, Star Trek has managed to create a fictional universe of such scope that it takes a while to orient in it. The franchise introduced countless alien species, so even though it still centers on humans, the fans have had the opportunity to meet different cultures.
Some of the most appreciated episodes are the ones that led the heroes to alien planets, which they would explore in greater detail. The most interesting planets in the Star Trek series have it all - from a detailed backstory to a unique culture that keeps the audience fascinated by it.
5 Risa
Risa is best known as the Pleasure Planet - and the name doesn't lie. Starfleet crews often go there for a vacation to get some rest and a breath of fresh air. Risa offers beautiful nature, beaches, gardens, pools, blue sky, trees, and simply everything the tired Starfleet officers need in order to get rid of the stress they feel. What's more, Risa attracts a lot of different cultures from all over the quadrant, making it an ideal place for studying alien species and coming in contact with them.
Of course, no paradise is without a flaw, as the Starfleet officers also learn. For example, when Captain Archer and his crew visit Risa in the series Star Trek: Enterprise, two of his men are tricked and robbed by local thieves. However, for the most part, Risa's beauty is enough to overlook this small detail.
4 Vulcan
The planet Vulcan is one of the most prominent ones in the franchise. Not only are Vulcans one of the founding members of the Federation, but the planet is also the birthplace of the iconic Mr. Spock. It's a direct opposite to the likes of Risa, since Vulcan is primarily a desert planet, making it a difficult place to live for non-Vulcans due to the high temperature and gravity. However, there's no denying the planet is visually appealing, and it has a long and interesting history.
Before the Vulcans became a peaceful species that relied on logic and suppressed emotions, the planet saw a lot of battles and wars. Even these days, some customs of the Vulcans might be considered barbaric by others, for example, sending out their children to the desert to survive on their own as a part of a test. In the Abrams movie universe, the planet was destroyed by the vengeful Romulan Nero, but it continues to thrive in the TV Star Trek universe.
3 Andoria
From hot to cold, that's also how to describe the difference between the planet Vulcan and the home of the Andorians. Andoria is an ice planet, and as such, the temperature doesn't cross over zero even in summer. In other words, whenever outsiders want to visit it, they should pack a lot of warm clothes. Visiting Andoria means taking on personal risk, however, as the Andorians aren't the friendliest hosts out there.
They're paranoid, prone to violence, and hold a strong grudge against other species, especially the Vulcans. Perhaps that has something to do with the fact that these two planets aren't that far apart, especially considering the vastness of space. Whatever the downsides are, Andoria simply looks cool (pun intended), and exploring the local mountains, as well as underground caves, are just two of the ways how to stay entertained there.
2 Sigma Iotia II
Andoria isn't the only planet whose visitors are risking their lives. Sigma Iotia II isn't as known, but that doesn't make it any less fascinating. The planet has a specific theme to it - the gangster theme, specifically; it all looks like Chicago during the Prohibition Era in the 1920s. Kirk and Spock learn this the hard way when they visit the planet and have a hard time blending in, especially Spock. Still, it's a better option for a visit than the Nazi planet, which also appeared in Star Trek: The Original Series.
And how did Sigma Iotia II get to this theme? It happened when the locals got the book Chicago Mobs of the Twenties and decided to stylize their society according to it. That means a lot of stylish suits and old cars, but also rough talk and guns, making Sigma Iotia II one of a kind among the Star Trek planets. It's a bit similar to the theme park in Westworld, but with live people.
1 Beta Omicron Delta III
Another lesser-known name but a fascinating planet, it allows the visitors' dreams to come true. The planet serves as a live amusement park that reads the visitors' thoughts and wishes and manufactures whatever or whomever they wish to see. That's a double-edged sword, as those who aren't careful can find themselves facing their worst nightmare or dangerous creatures. For example, Captain Kirk encountered his school bully there, and Doctor McCoy saw Alice and the talking white rabbit.
The good news is that as long as the visitors control themselves and what they're thinking about, they can experience something unique that would be impossible anywhere else. As such, it's a good place for a shore leave, even though Captain Kirk didn't agree with this assessment at first!