Star Trek: The Motion Picture actually WAS an episode of the TV series – it was a script that was written for the planned Star Trek Phase II series. So in that respect it is the most like an extended episode.
The Trek films are interesting, in that very few of them are “like” an episode of the series the preceded it – Almost all the Trek films use tropes and plot points from the series they were based on, but the characterizations and plots are heightened for an audience that are not hard core “fans” – As much as I like The Wrath of Khan, I have to admit it has very little “in common” with the original series… and that is a good thing. Khan has changed… Kirk, Spock and McCoy has changed. The characters are more mature, and this maturity made a god story even better.
In the end, the needs of a film are different than the needs of a recurring TV series… and the filmmakers behind the Trek films have accepted and acted upon these differences… to varying degrees of success.