One could argue that Garak is quite interesting. He's a Cardassian exile with a mysterious past. He's full of secrets and his interactions with the other characters are always filled with intrigue. And Jadzia Dax is also great. With her symbiont's memories and experiences, she brings a wealth of knowledge and a different perspective to the crew.
Sisko is really interesting. He's a complex character, being a single father and also a Starfleet officer in charge of Deep Space Nine. His relationship with his son adds a human touch to his role as a leader. Then there's Quark, the Ferengi bartender. His constant scheming for profit and his unique view of the universe make him stand out.