Dragon Ball Z Movie 6: Cooler Returns is a sequel to Dragon Ball Z Movie 5: Cooler's revenge. So unless you don't want to get spoiled about stuff from the previous movie, I suggest watching the previous one first.
It is the first movie where Piccolo doesn't appear to safe Gohan since that cliche was introduced. But it is also the first movie Vegeta appears and seeing up his role as "Only I can beat Kakarot." as he stopped the villain from killing him. And its the first time Vegeta and Goku actually work in unison to beat a bad guy.

As the name suggest this movie is about Cooler reappearing Frieza style. Tho not to take revenge on Goku but to invade New Namek which looks like regular Namek for some reason. Well, you can tell this movie was made before the Boo Saga so the writers didn't know how New Namek actually looks like.
In this continuation which takes place around the time of the Cell Saga before the use of the time chamber, Dende is the new God of Earth earlier and requested from Goku and the gang to go to New Namek because of shit happening there.
We have Goku, Gohan, Piccolo and Krillin as fighters. Roshi, Yajirobe and Oolong for comic relief and who want to explore I guess. Yajirobe only exist obviously because he was ordered by Korin, the god of martial arts as he has the Senzu.
On Namek the Namekian are enslaved by Super Robots from a giant squid like planet that eats planets to get mightier. Goku and the gang arrive to stop them and pretty much struggle against these Robots. Normally I am against some robots posing a threat to the Z Fighters, but we will find out that the machines are created with the energy of god knows how many planets the BIG GETE STAR consumed. So they stronk.

Amounts the robots there is obviously Cooler who was saved from being incinerated from the Sun fighting Super Saiyan Son Goku. He obviously is happy that Goku came for a rematch saving him the trouble to look after him.

This movie uses its cast far better than the awful Revival of F movie. The foot soldiers are actually posing a threat and Goku is kept busy fighting Cooler. While in Revival of F Frieza didn't lift a finger and made it feel like the movie time is being padded.

Many complain about this movies art quality. While it looks bad in some scenes and if you pause during action scenes, the fights if you watch it is very well animated. When it comes to action, very few Dragon Ball movies can keep up with Return of Cooler.

This movie is also good at setting the tone of hopelessness. There are several times where it seemed like the character's trump card finally worked only for it to fail or something more dangerous to happen. An example is when Goku finally decided to turn Super Saiyan looking like he got the advantage and delivered a major blow. Only for the Gete Star fixing Cooler in a way such a strike won't work twice.

This made Cooler's regenerative powers unique amounts other Dragon Ball villains and honestly his responses to anything thrown at him. The film is also really good at justifying machines being able to fight the Z Fighters. While I don't like the Androids from Gero, minus Cell, being able to fight anyone at Frieza's tier, I can buy Cooler and the robots being able to fight them. Considering one of the robots is Frieza's older brother who required a Super Saiyan to put down and the Gete Star having energy of several planets and its lifeforms in it.
It really feels like the common man vs the industry. So there are some interesting symbolism that goes more than 2 guys punching each other until the other is down.

Finally what brought down Cooler and the Gete Star also had a great symbolism where human have things like willpower and spirit aka there is more to men than flesh and blood.

Besides the comic relief cast, each member was crucial. Without Vegeta, Goku would likely not be able to beat Cooler. Unlike in many other movies where he feels like he exist because he is popular like in Dragon Ball Movie 13 Wrath of the Dragon or Dragon Ball "Z" Movie 15 Revival of F. That is because in most movies he gets beaten up and his fight don't really have any consequences to the outcome.

Criticism
There are also some negatives. For instance Goku still using the Kaioken despite it being outdated when he can just turn Super Saiyan. The comic relief trio being kinda irrelevant considering Krillin and Gohan were also kinda filling that role. Not that I personally hate the scenes since I really love me some Master Roshi. Especially being in a movie without any "hot chicks" for him to fall over for. So you don't get the same repetitive jokes.
One last scene that bothered me is when Vegeta and Goku seemed like they finished off Cooler only for them being surrounded about to enter another fight. And that is when Goku flat out states the obvious in the scene with a great scene where they perfectly worked as "Show don't tell".
And obviously some scenes looked godawful where many use this movie as means to defend Dragon Ball Super's early shitty animation. I also feel this movie could have worked without Cooler being the villain since the Big Gete Star was already justifying its prominence as a threat. So this felt like the writers were cashing in on the popularity of a villain. Tho it's not as bad as Frieza returning for a billion times and Broly keep reappearing in various forms.
Lastly Piccolo, despite being the strongest at this point, because he fused with God, didn't do as much as you would hope. The Super Saiyans pretty much overshadowed him in this movie.

Overall this movie was very good. Despite its negatives it pretty much shits on most Dragon Ball movies in terms of writing and quality. The soundtracks in Japanese are used amazingly and the visuals feel like its an homage to old scifi anime. So if you like these kinds of things, you will definitively fall in love with his movie.
And like Cooler's Revenge, it is highly recommended to watch it even if its "non-canon".