While comparing Star Wars to Star Trek is tempting considering both are long-running franchises with spaceships, it is like comparing apples to long division when you get down to brass tacks. Any commonality they might have is what they would both have with any story. In other words, comparing Star Wars to Star Trek is like comparing Star Wars to Titanic.
SIDE NOTE: On which Trek films are worth watching, of the original six there are two ways to watch it. There's the "trilogy" consisting of 2-4 and following the 'odd numbers suck" rule which would be 2, 4, 6. Both are valid ways to watch several Trek movies.
Star Trek II: the Wrath of Khan is the best film out of the original ten. So if you watch only one, this would be the one to watch.
Star Trek III: the Search For Spock is a weaker entry in the series but necessary as various plot points are covered here and later films would be confusing. It's not a horrible film. It's just not as good as some of the others.
Star Trek IV: the Voyage Home is kind of an odd duck as it's basically a time travel comedy with Star Trek characters. Not that humor was a stranger to the original series ("Trouble With Tribbles," anyone?) It's unique in that there is no central villain and the plot manages to give most of the characters a chance to have the spotlight as they try to deal with the modern day (circa 1986). It is probably the most mainstream Star Trek movie because of this.
Star Trek VI: the Undiscovered Country is basically a whodunnit in space. It's not quite the waste of major events or themes of Star Trek III, but it's not up to the level of Star Trek II. But as the final voyage of the original crew, it is enjoyable fare.
I skipped Star Trek: the Motion Picture and Star Trek V: the Final Frontier because these movies, in a word, suck and should only be watched by fans. If you try watching I or IV and become a Star Trek fan and simply cannot get enough, then give these movies a shot, and only give V a shot if you really, really need a Trek fix.
Star Trek: the Motion Picture suffers for several reasons. One, it was the late 70's so the uniform redesign is awful. Words can't do it justice. Two, well the story in general has multi-faceted problems. It's padded out from a television script for a failed Trek series. It might have worked for an hour with commercial breaks, but not for two hours. Besides, the basic concept was already done in the original series episode "The Changeling." As such, I can only recommend it to people with a great deal of patience as the film is not nicknamed "Star Trek: the Slow Motion Picture" for nothing. But it's not a total loss. The conflict between Admiral Kirk and Commander Decker works even though it has no real payoff. Just most of the film has nothing going on and much of the stuff that does happen winds up being a dead end. So I can only recommend it to devoted Star Trek fans. But if you are a devote Star Trek fan, you've probably seen it already.
Star trek V: the Final Frontier probably needs no explanation as even people who have never seen anything Star Trek have heard that this one is really, really bad. And it is bad. The main problem is it was a vanity project by Shatner as a love letter to his character. However, in order to make Kirk look good, he wound up making everyone else look stupid. Couple this with the plot he had in mind-- the search for god--- and the studio's decree that it should be a comedy like part IV which was the highest grossing in the series at the time. These two ideas did not mesh together well. The end result is a film that's just all over the place and doesn't seem to say anything and what little it does say is incredibly stupid. So, I can't really recommend this one to even Trek fans since it is such a failure and it takes liberties with the series conventions that I don't think any Star Trek fan would appreciate.
As for the Next generation films, I say skip them. I only watched them up to Star Trek: First Contact which I'm told was the last good one as last two films have both gotten a consensual thumbs down, and I hated First Contact. I thought it was stupid and I remain baffled that it is considered good by anyone.
These, of course are just my opinions on the Star Trek film series. Don't just take my word for it.