I think to be thorough and fair, we can't stop at just asking which series had the most time travel episodes. There should be several criteria that show how time travel is used. For instance, if we were to ask if time travel resolved the problem posed by the episode, DS9 might not come out ahead. PAST TENSE, PART I and II: No, Sisko must replicate conditions of Bell Riots. VISIONARY: Yes, O'Brien obtains information from the present. THE VISITOR: Yes, brings characters back to the accident. LITTLE GREEN MEN: Maybe, escaping captivity and leaving planet were sufficient. TRIALS AND TRIBBLE-ATIONS: No, crew had to stop assassination of Kirk. CHILDREN OF TIME: No, the time travelling event was nullified. WRONGS DARKER … : No, Kira was the cause of the problem (attempted assassination),which she resolves herself. TIME'S ORPHAN: Yes, Molly encounters younger self. THE SOUND OF HER VOICE: No, character is already dead. As many as four of the nine episodes needed time travel in order to resolve the conflict. Compare that to Enterprise, where only 2-3 of the 14 episodes made time travel the tool for resolution: Shockwave, Twilight, and maybe Carpenter Street. I think other questions could be asked to get more into which series used, or abused, time travel.