Star Trek has brought joy and amazement to fans for decades. The franchise's pinnacle may have been in the 90s, a decade that saw three different series on the airwaves. While two of these series focused on space travel, Star Trek: Deep Space Nine shifted focus to a space station. This new setting, coupled with the darker tone, changed up the franchise's formula in more ways than one.
Having three Star Trek series on the air during the decade presented the opportunity for crossovers between the series. Using alternate dimensions and time travel, Deep Space Nine characters found their way on to one of the other series. Plus, modern Star Trek has found a way to bring back classic characters.
5 Julian Bashir: The Next Generation
The starship Enterprise docked at the Deep Space Nine space station setting up this crossover in the episode "Birthright Part 1." In the episode, Data has a dream-like experience after an accident, and it causes him to hallucinate about his creator. While searching for the meaning behind this dream, Data turns to some help from both the Enterprise and Deep Space Nine crews.
Julian Bashir is the Chief Medical Officer on board the space station, so Data works with Bashir and Geordi La Forge to recreate the circumstances that led to his dream. Both Bashir and Geordi are reluctant but still help Data with his experiment. This was Bashir's only appearance on the other Star Trek program.
4 Quark: The Next Generation, Voyager, Lower Decks
Quark has the widest range of crossovers of any Deep Space Nine character, appearing in three other series. He makes a very brief cameo in The Next Generation in the episode "Firstborn," providing information to Riker. Quark also appears in "Birthright Part 1," the same episode that featured Julian Bashir. Finally, he briefly appears on Voyager in the series premiere, "Caretaker." The Voyager is leaving from Deep Space Nine, and Quark appears at the beginning of the episode attempting to swindle the naive Harry Kim.
Quark makes his animated debut in the series Lower Decks. In the episode "Hear All, Trust Nothing," the crew of the Cerritos is docked at Deep Space Nine, and Lower Decks crew members stop by Quark's bar. It's revealed that Quark stole parts for his replicators, and a deal must be reached for the alien whose parts were stolen to get part of Quark's profits.
3 Morn: The Next Generation, Voyager, Lower Decks
Alongside Quark, Morn makes an appearance in the same episodes of Voyager and Lower Decks, since he frequents Quark's bar. He also has a cameo in The Next Generation. Morn was always a popular background character on Deep Space Nine, often seen in the background of scenes in Quark's bar. He continued this tradition in his appearances on other series.
These appearances put Morn in the company of Quark, Picard, Riker, Troi, Kang, and Q, who all appeared in multiple series across the franchise.
2 Kira Nerys: Lower Decks
Following the series finale of Deep Space Nine, Colonel Kira Nerys took over the space station. That continues in her appearance in the Lower Decks episode "Hear All, Trust Nothing." She appears in the episode giving the bridge crew of the Cerritos a tour around the space station, while the Lower Decks crew are on their own journeys and visiting Quark's bar.
It's through Kira that Deep Space Nine fans get the Easter eggs back to the series they love so dearly. Kira shows off Sisko's baseball, letting fans know she is still awaiting his return, and is ready to hand the office back over to the Captain who commanded the station for years. While Quark's appearance could have been the plot to any episode featuring the Ferengi, and Morn sat in the background like he always does, Kira's appearance carries the torch for the rest of the Deep Space Nine crew.
1 Worf: The Next Generation, Picard
Michael Dorn has the most single appearances in Star Trek history, so it makes sense that his character, Worf, would have multiple crossover appearances in some of the wildest ways possible. Worf started as a crew member aboard the starship Enterprise during The Next Generation. The Klingon rose through the ranks on the bridge crew, becoming the security officer following the death of Tasha Yar. He was an important member of the crew for all seven seasons before leaving the Enterprise to join the Deep Space Nine crew after Enterprise was decommissioned.
Worf would also appear in all four films featuring The Next Generation crew. While those appearances have raised some questions about continuity, his appearance in Nemesis addressed his fate after Deep Space Nine, where he was returning to Starfleet after serving time as a Klingon ambassador. Worf would also make appearances in season three of Picard. He appears as a member of Section 31 who is now Rafi's handler. He works with her to uncover the Borg's plans and then works with his former crew to stop the Borg and save the United Federation of Planets, again.