Which of the original series and next gen era Star Trek movies do you think is the best, and what makes it the best?

I really like Undiscovered Country. It captures the time it was filmed in very well. It has all the good bits of star trek and some good bits from star wars on top. Above all, it has a fully fleshed out beginning, middle and end. Exactly like a story should be, well balanced. Running time is under 2 hours, but it feels like it should be a lot longer for all that happens.

Wrath of Khan is next as the best submarine movie, then First contact.

If I had to rank them from best to worst:

Wrath of Khan
First Contact
Undiscovered Country
Voyage Home
Search for Spock
Generations
The Motion Picture
Final Frontier
Insurrection
Nemesis

The first four are great classic sci-fi films. SfS & Generations were decent. The rest were mediocre-to-bad.

I almost agree with your ranking. I'd move Insurrection and Nemesis above TMP and Final Frontier. Insurrection felt like a two-parter for TNG with its low budget. But overall I'd rather watch it over Nemesis, which I'd rather watch out TMP and Final Frontier. Though Final Frontier does have the best line in movie history. "What does god need with a spaceship?" 

1. Wrath of Khan
2. First Contact
3. Undiscovered Country
4. Voyage Home
5. Search for Spock
6. Generations
7. Insurrection
8. Nemesis
9. The Motion Picture
10. Final Frontier

My personal ranking of all Trek movies:

1. The Voyage Home
2. First Contact
3. Wrath of Khan
4. Undiscovered Country
5. The Search For Spock
6. Generations
7. Nemesis
8. Insurrection
9. The Final Frontier
10. The Motion Picture

And though this thread is only about the TOS and TNG movies, I would put all 3 of the JJ-verse Trek movies between Insurrection and Final Frontier personally. While JJ-verse is definitely not Star Trek imo, really a complete bastardization of everything that makes Trek "Trek", they are good dumb fun summer blockbuster type movies and I enjoy them more than the nonsensical plot of Final Frontier and the snoozer that is The Motion Picture. 

Voyage Home and TMP are the only films that remain true to what Trek was about. I like WOK but it also ruined Trek films because nearly every single Trek film after became bad guy wants revenge. Defeat bad guy in the big battle at the end. Wrath was better in its day but diluted by all the copycats since. Voyage Home is the most original idea. The most fun. The most like Trek.

The voyage home never grabbed me. Tomorrow is yesterday and The City on the Edge of Forever are both much better. It was a charming movie, just didn't feel very original to me. I enjoyed it more when rewatching it at an older age, especially since I had been to San Francisco in the mean time. Much better when you recognize all the landmarks.

Going back into Earth's past is way overdone by now, Quantum leap, Dr. Who, Travelers, Timeless, so many movies, every star trek season. It's about time we got another planet to travel back in time to :D Or you know, could go into the future.... like sci-fi! Hence First Contact before Voyage home for me.

I honestly wasn't sure where to put TMP. It's not a bad ST film per se, but it is boring, being way too slow and cerebral in a way Trek usually isn't. It does have great effects and music, and the scene giving a detailed introduction to the remodeled Enterprise was beautiful (it's still my favorite version of that ship).

Final Frontier was a very hokey film with sometimes questionable effects, but it has enough enjoyable parts to keep it from the bottom of the list. Kirk, Spock, and McCoy bonding during a camping trip. The stuff with the Klingons. The gag where Scotty bumps his head. And of course the scene you mentioned.

Insurrection meanwhile is an utterly forgettable film, and Nemesis was a terrible way of closing out the series. The TNG crew deserved better than that as a sendoff.

Undiscovered Country is the best Trek film from any era imo.

Since others have posted rankings, I'll post mine too:

1. Star Trek VI: The Undiscovered Country - 8.5/10
2. Star Trek IV: The Voyage Home - 7/10
3. Star Trek II: The Wrath of Khan - 7/10
4. Star Trek: Generations - 6/10
5. Star Trek: First Contact - 5/10
6. Star Trek: The Motion Picture - 5/10
7. Star Trek III: The Search for Spock - 5/10
8. Star Trek: Insurrection - 4.5/10
9. Star Trek V: The Final Frontier - 4/10
10. Star Trek: Nemesis - 3.5/10

I'm not a huge fan of the movie series; I much, much prefer the TV shows (although not Picard or the most recent season of Discovery, they're atrocious).

Here’s my list, I love Star Trek lol, i really wish they’d make a good Star Trek game.

1. First Contact
2. Wrath of Khan
3. The Voyage Home
4. Undiscovered Country
5. The Search For Spock
6. Generations
7. Insurrection
8. Nemesis
9. The Final Frontier
10. The Motion Picture

I feel both First Contact and Wrath of Khan have the most movie feel to them, they feel big, visuals are great, story’s are on point. They don’t feel exactly like a Star Trek episode but I expect that from a big budget film.

My only issue with First Contact is the scene that Lily goes in and talks Picard out of hunting down and killing the Borg, I believe that should have been Crusher, would have made more of an impact based on their past relationships.

If we included the JJ movies, I’d put them all above generations and below undiscovered country.

It’s a shame insurrection didn’t get the reshoots and space battles they wanted to put in after filming to make it a better movie. They’d rather push it out the door they make it a better movie.