It doesn't have to be every season of every series, but it can be. Some of these choices of mine are pretty firm but this is about as accurate as what I feel at the moment in most cases.
TOS-1: "The City on the Edge of Forever"
TOS-2: "The Doomsday Machine"
TOS-3: "The Enterprise Incident"
TNG-1: "1001001"
TNG-2: "Q Who?"
TNG-3: "The Best of Both Worlds, Part I"
TNG-4: "Reunion"
TNG-5: "Cause & Effect"
TNG-6: "Frame of Mind"
TNG-7: "Lower Decks"
DS9-1: "Duet"
DS9-2: "The Circle"
DS9-3: "Past Tense, Part II"
DS9-4: "The Way of the Warrior"
DS9-5: "Call to Arms"
DS9-6: "In the Pale Moonlight"
DS9-7: "The Changing Face of Evil"
VOY-1: "Eye of the Needle"
VOY-2: "Projections"
VOY-3: "Macrocosm"
VOY-4: "Scientific Method"
VOY-5: "Counterpoint"
VOY-6: "Tinker Tenor Doctor Spy"
VOY-7: "Lineage"
DSC-1: "Lethe"
Some of these could change at a moment's notice. If I typed this list again, it wouldn't come out the same. In some cases, it was hard to narrow it down to just one.
TOS-1: "The City on the Edge of Forever"
TOS-2: "The Doomsday Machine"
TOS-3: "The Enterprise Incident"
TNG-1: "1001001"
TNG-2: "Q Who?"
TNG-3: "The Best of Both Worlds, Part I"
TNG-4: "Reunion"
TNG-5: "Cause & Effect"
TNG-6: "Frame of Mind"
TNG-7: "Lower Decks"
DS9-1: "Duet"
DS9-2: "The Circle"
DS9-3: "Past Tense, Part II"
DS9-4: "The Way of the Warrior"
DS9-5: "Call to Arms"
DS9-6: "In the Pale Moonlight"
DS9-7: "The Changing Face of Evil"
VOY-1: "Eye of the Needle"
VOY-2: "Projections"
VOY-3: "Macrocosm"
VOY-4: "Scientific Method"
VOY-5: "Counterpoint"
VOY-6: "Tinker Tenor Doctor Spy"
VOY-7: "Lineage"
DSC-1: "Lethe"
Some of these could change at a moment's notice. If I typed this list again, it wouldn't come out the same. In some cases, it was hard to narrow it down to just one.