Which Star Wars movies are definitely worth watching ??

Star Trek or Wars? I only ask because you typed in Trek, but you're on a Star Wars forum.

If Trek:
I like the Motion Picture, most people think it's too slow.
Wrath of Khan is gangbusters (awesome)
Voyage Home is funny
Undiscovered Country is really good, too.
First Contact is a quasi-zombie picture in space - worth checking out - good action
I kinda liked Insurrection...most didn't.
Star Trek (JJ Abrams') is great fun.

If Wars:
Star Wars (Episode IV: A New Hope) is my favourite
Empire Strikes Back is most people's favourite (I love it, too!)
Return of the Jedi is good fun, gets a little silly at times, but has a pretty epic climax between Luke and Vader.
The Force Awakens is a killer action picture.

Honestly, if you've seen the first one, A New Hope, you got the gist of the whole Star Wars universe. The rest is just filling in the details.

I've never watched Episode 3 and Episode 4 back to back as just a pair. I always thought that would be interesting.

People love Empire, but it's just a fragment of story. I mean it's a great film, but it starts in the middle of a story, it ends in the middle of a story. And because the story doesn't go anywhere brilliant in Return, what's the point of even getting invested?

I've been meaning to watch Rogue One again. Maybe i'll make a trilogy out of Episode 3, Rouge One, and Episode 4. That could be fun.

A New Hope , Empire Strike Back , Return of The Jedi , Revenge Of The Sith , Rogue One , The Force Awakens

The Force Awakens and The Last Jedi.

The original Star Wars and The Empire Strikes Back. Jedi has a few good scenes, but it's (for the most part) goofy as can be. Rogue One had some good scenes (mostly in the way that they used old footage from the original Star Wars)everything else pretty much stunk. I will say that the prequel trilogy had some GREAT ideas, but a lot of the acting and directing stunk. Darth Maul was an amazing character and Ewan McGregor was awesome as Ben Kenobi