While the position of a lieutenant commander doesn't draw as much attention as the rank of a captain in the world of Star Trek, it's still a highly valued position that not everyone can reach. In fact, it often takes Starfleet members years, if not decades, to become lieutenant commanders, and some never manage it.
Throughout the history of Star Trek, multiple more or less-known characters had this rank. Some of them became popular among the fans and played a significant part in the story. However, only one lieutenant commander of the Star Trek universe can be the best.
5 Jadzia Dax (Deep Space Nine)
Jadzia Dax served as a lieutenant commander on the Deep Space Nine space station. Even though Jadzia looked like a young woman, thanks to the Trill symbiont that was implanted in her, she had the experience of centuries of lives lived by other Trill. This provided her with unique insights, and helped her remain calm even in situations when other crew members were losing their heads.
Jadzia's intelligence, composure, and sense of humor all made her one of the most popular characters in Star Trek: Deep Space Nine. Through her, the fans got to learn more about the Trill species as well as the rules and details of their fascinating society. Unfortunately for Jadzia, even her knowledge and experience didn't save her from dying in the sixth season. She was subsequently replaced by Ezri, who was much less popular among fans.
4 Tuvok (Voyager)
Serving in Starfleet was always a challenge for the Vulcans due to their philosophy that differed greatly from humans, the dominant species in Starfleet. So when Tuvok was trapped alongside the rest of the crew of Voyager in the Delta Quadrant, being separated from his people was difficult for him. Despite that, Tuvok's expertise and extensive knowledge proved instrumental for the crew in returning to the Alpha Quadrant and their friends and families.
Tuvok worked as an instructor at the Starfleet Academy and helped thousands of cadets become better at their studies. He and Captain Janeway became close friends and Janeway relied on Tuvok's advice for many years, even trusting him to go undercover with the Maquis despite the fact that most Vulcans abhor lying and avoid deception at all costs (unless it suits them or the situation demands it).
3 Leonard McCoy (The Original Series)
As the chief medical officer in Captain Kirk's Enterprise, Leonard McCoy had the attitude of an old country doctor, as he said. Despite that, he proved efficient at working in space and curing unknown diseases, as well as helping alien species the likes of which he hadn't encountered before. Even though McCoy often accompanied his Captain and Spock on the landing parties, he was a scientist at heart, not an explorer, and spent as much time in the labs and in the medical bay as he did outside the ship.
Not only did he help save countless lives during his time on the Enterprise, but was also considered an excellent space psychologist. Unlike many less fortunate Enterprise crew members, McCoy led a long life, became an admiral in the end, and even got to visit Captain Picard's Enterprise and see its medical facilities before the ship went on its journey.
2 Data (The Next Generation)
As one of the most accomplished Starfleet officers, Data also turned out to be an excellent lieutenant commander. As an android, Data had access to much more information than any human could remember. Not only was Data super intelligent and educated, but he was also physically stronger than the human crew members on the Enterprise.
Of course, even Data wasn't perfect. He struggled to comprehend social intricacies and expressions, especially in the earlier seasons of Star Trek: The Next Generation. However, his strengths prevailed over his weaknesses. Data managed to be not only a valued member of the crew but also befriended most of them, including his captain, Jean-Luc Picard. Data's effort to become more human makes the development of his character even more interesting to watch.
1 Spock (The Original Series)
Considering his popularity among fans and the long and prosperous life Spock led across countless Star Trek TV shows and movies, he remains the best-known, best-loved lieutenant commander of the Star Trek universe. That's despite the fact that he could have easily walked a different path and join the Vulcan Science Academy. Instead, Spock joined Starfleet as the first Vulcan-human in history. He rose through the ranks swiftly, serving both underneath Captain Christopher Pike and Captain James T. Kirk.
Spock's tendency to think things through made him an excellent first officer to the more impulsive Kirk. Spock deployed logic to help him solve situations that would have left many other officers scratching their heads. And when it came down to it, he was even willing to die for his ship. In the end, Spock went on to become an ambassador and fought to restore the unity between the Vulcans and their more violent cousins, the Romulans.