Dragon Ball Z movie 1: Death Zone is a movie that can be watched before even starting Dragon Ball Z, but it requires finishing the original show if you want to know who the characters are.

The story is about the son of the God of Earth's former rival Garlic Jr who wants revenge for his father's defeat and create a world for demons. To do that, he and his henchmen ambush Piccolo who is God's evil counterpart to indirectly eliminate him since the lives of God and Piccolo are connected. Because of circumstances, the main character's son has been kidnapped too.
It is up to Goku, Krillin and Piccolo to stop Garlic Jr and save Gohan.

It is a simple story with a rather unique premise for Dragon Ball. This is the only Dragon Ball movie God is involved and one of rare accessions he is involved in combat. Not to mention, we get more insight about his past and Garlic Jr doesn't feel like an antagonist that came out of nowhere like many other Dragon Ball antagonist.

Visually this movie strikes as unique since in terms of presentation it really feels like a movie and not like an extended episodes. Even the use of colors are unique for Dragon Ball which frequently uses bright colors such as blue and green and fights are in daytime. In this movie, they use more eerie colors like reddish orange for the sky and a more darker color pallett on the fighting stage and antagonists giving a more demonic or scary feel.

Action and humor are Dragon Ball's bread and butter and Death Zone combines them well since it is the transitioning period between Dragon Ball and Dragon Ball Z, it can combine them excellently. Gohan is often used for the comic relief scenes but even in combat we see these when Goku fights Garlic Jr's henchmen which displays great use on weapons too which were useful back then.
This movie is also the first time two characters who are enemies put their differences aside and work together. That being Goku and Piccolo showing how much of a threat Garlic Jr is that even the strongest characters up to this point have to work together.

However there is some criticism. While many Dragon Ball Z movies don't fit the main story it doesn't hurt ignoring them or seeing them as what ifs. But if you want to watch Dragon Ball Z with filler and all, you are required to watch this movie since it will play a role in an arc of Dragon Ball Z. Meaning that not being able to explain the inconsistencies between anime series and film can be off putting since the characters meet Gohan in Z the first time but this movie can only work before watching Z in terms of fitting into the main story timeline of the anime adaptation of Dragon Ball.
The last criticism is how the badguy is defeated because he is at fault mainly and another point might be considered a Deus Ex Machina which did get build up in the movie. But it isn't too bad when you watch or have watched Dragon Ball Z since it happens often and is someone's character trait that will get a payoff at some point.

Overall this movie is good. Little short and lacks the transformations of the good guys many fans are hoping from Dragon Ball, and the main antagonist might not be that memorable, it still is an experience you will enjoy.