For the original series movies... The Motion Picture (#1) is basically an elongated TOS episode. Doesn't really influence any of the other films. Some enjoy

For the original series movies... The Motion Picture (#1) is basically an elongated TOS episode. Doesn't really influence any of the other films. Some enjoy it, some find it dull. The Wrath of Khan, The Search for Spock and The Voyage Home (#2-4) form a trilogy and must be watched in order. The Final Frontier (#5)...I loved it when I was nine. Skippable otherwise. The Undiscovered Country (#6) should be watched after #2 through #4. It and The Wrath of Khan are typically tied for the favorite Trek film.