Dragon Ball Z follows the story of the legendary warrior Son Goku, a Saiyan from Planet Vegeta who is originally sent to destroy Earth (at least, prior to Dragon Ball Super: Broly) but actually ends up saving the planet from villains with the power to pop planets as if they were pimples. In this world filled with fierce energy attacks and intimidating transformations, there are hundreds of techniques that fighters learn to give them an edge in battle. There are also many techniques that do the exact opposite.
Because of the sheer number of techniques used in Dragon Ball, Dragon Ball Z, Dragon Ball GT, and Dragon Ball Super, this article will only cover techniques used in Dragon Ball Z (including movies and filler episodes). An analysis of techniques in Dragon Ball and Dragon Ball Super will come in due time, especially as most techniques from Dragon Ball Z's predecessor can be pretty niche and obscure. Looking at you, Monster Carrot.
10 Useful: Body Change
Captain Ginyu's signature move was first displayed during his battle with Goku on Namek, allowing him to masquerade as the Saiyan warrior to trick and disturb his friends for a short period of time. This technique has many perks, the main one granting Ginyu the power of those he exchanges bodies with, even if they are significantly stronger than him, not to mention that his injuries do not transfer from one body to the next.
The downside to this technique is that it requires him to yell "Change, now!" Therefore, forget about catching an opponent off guard with this move, especially if you're playing Xenoverse 2.
9 Useless: Game Boy Bomb
Yes, Mr. Satan's moves count as techniques. While creative, this particular one wasn't effective in the slightest against Majin Buu. To defeat his powerful pink opponent, he gave Buu a game, which he rigged to explode. Against a normal human, maybe even someone like Yamcha or Chiaotzu, this technique would surely send them to King Yemma's.
Against supernatural fighters like Buu, Cell, Frieza, and even Goku, a handheld bomb would barely be able to scratch them. Not even the game that Majin Buu was playing on the device looked interesting.
8 Useful: Destructo Disc
Krillin often gets a bad rep due to the number of defeats he has suffered, but he should be given more credit for inventing one of the most versatile attacks in the entire series. If not for Vegeta's warning, Krillin's Destructo Disc would have killed Nappa, and it cut through Frieza's tail like butter.
Other characters have also appropriated the technique for their own needs: Vegeta used it to slice through Meta-Cooler's arm, Android 18 used the disc to separate Goten and Trunks when they were disguised as Mighty Mask, and Goku used the attack to slice Super Buu in half, preventing him from killing Gohan. Though it has never ended a battle, the Destructo Disc has given fighters a chance against superior opponents.
7 Useless: Super Speed Dash
Super Speed Dash was used in the Other World Tournament by a petite fighter named Tapkar from the West Quadrant. In his match against Torbie from the South Quadrant, Tapkar showed off his signature technique from the start, dashing all around the arena with such speed that even Goku had trouble keeping up with him.
Unfortunately, the technique drained all of Tapkar's energy and he was forced to give up the fight due to exhaustion. Fans were never given an explanation of what exactly killed Tapkar, but you can bet he was probably out of breath when he died.
6 Useful: Kaio-ken
Before Goku awakened his Super Saiyan powers, the Kaio-ken was one of his best aces. Serving as a multiplier to his strength and senses, Goku often used this attack in short bursts against opponents who outclassed him. The Kaio-ken enabled his Kamehameha to overpower Vegeta's Galick Gun during their first fight, and it gave him the speed he needed to intercept Nappa in mid-air before he could hurt Krillin and Gohan.
Although Goku is the only person to have used this attack successfully, King Kai can teach it to anybody. The major downside is that this technique takes an immediate toll on the user's body, and there are even fan theories that the use of this ability is what gave Goku the heart virus.
5 Useless: Down Burst
After losing an arm to Androids 17 and 18 in an alternate timeline, Gohan was forced to rely on one-handed techniques. 17 and 18 charged at him at the start of their final battle, prompting him to use his Down Burst technique in an attempt to disorient them while positioning himself in the air above them.
The reason this technique is useless is that there are other moves with that same effect (like Solar Flare or Instant Transmission). The smokescreen caused by this discharge of energy does no good against foes who can track his movements by following his energy.
4 Useful: Barrier
Depending on the user, this technique goes by many names and colors but functions the same throughout Dragon Ball Z. Android 17's barrier shielded him from Piccolo's Hellzone Grenade, and Future Gohan's barrier protected the Saiyan from a barrage of ki blasts from the Androids.
Majin Buu conjured up a barrier to shield himself from a military assault, and even King Vegeta used one during a flashback in the episode "The End of Vegeta." This move can be activated without placing an enormous strain on the user, and the barrier is particularly resilient to energy-based projectiles. As long as the opponent isn't Goku, there isn't a plot device in the world that can break through this defense.
3 Useless: Flooding Technique
In the Other World Tournament, a fighter named Arqua from the East Quadrant unleashed his signature technique on Goku, encasing the entire arena in a block of water. While fighting in the water did provide Arqua with a home team advantage, Goku merely jumped out of the water and fired a Kamehameha so powerful that it dried away the water, sending Arqua out of the ring.
Maybe if there was some sort of barrier (see above entry) around the water to keep opponents sealed in, this technique would become one of the most useful techniques in the entire franchise.
2 Useful: Instant Transmission
While Goku was the first character to demonstrate Instant Transmission onscreen, various Dragon Ball Z characters have added this amazing technique to their arsenal, including Cell, Meta-Cooler, and Kid Buu.
What makes this technique so valuable is that it only requires the user to lock onto a vague energy signature to travel thousands of miles in seconds. Instant Transmission can be combined with others to create fatal, disorienting attacks, the most famous one being Goku's Instant Transmission Kamehameha.
1 Useless: Angry Shout
Dragon Ball Z is known for its screaming, especially when characters are powering up or transforming. Fans will be shocked to know that screaming is actually a technique in this universe. Kid Buu's first act was to let out a scream that caused natural disasters all over the planet. Uub also used this technique, but after causing tiles of the World Tournament arena to rise up, he nearly knocked himself out of the ring after one struck him in the back.
Though the effects of an Angry Shout differ based on the user, at the end of the day, it's just a scream, and nobody's ever died because of a scream. Not even in Dragon Ball Z.