Dragon Ball is a franchise based on special moves. From Kamehameha Waves to Special Beam Cannons, everyone has a signature attack that defines their martial art and personality. Some characters seem to get more love than others in this regard, but it's undeniable that everyone has a little bit of flair to call their own.
Throughout the series, some of these attacks have proven to be more useful than others. What's strange is that even the most useful attacks don't seem to get enough screen time according to the fans. Sometimes the authors forget about the moves. Other times, the moves just aren't useful in a specific fight. Either way, the series would be a lot more interesting if some of these moves were considered more often.

Evil Containment Wave

Roshi doing the Mafuba in DBFZ
The Evil Containment Wave, or Mafuba technique is a sealing technique used to keep demons away from the human realm by sealing them inside a special container. The technique isn't difficult to learn compared to other techniques in the Dragon Ball universe, so it's strange that the only character seen using it is master Roshi.
The reason nobody else uses it could be because it takes a good portion of the user's life force to use, but the dragon balls are a good solution to that problem should the user die. The only other character who attempted to use the Mafuba was Tien, who tried way back in the original Dragon Ball series.

Kaio-ken

Goku Doing Kaio-ken
Kaio-ken allows its user to become stronger, faster, and have a higher capacity for ki control than normal, at the cost of extreme physical strain. King Kai taught the move to Goku, and he used it to defeat Vegeta and save planet Earth. Goku shows a high affinity for Kaio-ken, but only uses it a few times in the whole series, even though its benefits can always be reaped, even after achieving super saiyan status.
The last time Goku uses the Kaio-ken in the manga is against Frieza, which was literally decades ago at this point. It's fine to say that Goku has surpassed the need for the technique, but he has so many moves he doesn't use at this point, it might be more interesting to see a different character use them. Because of the iconic moments Kaio-ken appears in, many fans want to see it come back.

Instant Transmission

Goku preparing to teleport Cell away with instant transmission
The Instant Transmission technique is a staple of Goku's abilities and an iconic sound effect in the Dragon Ball series. Goku learned the move while on Yardrat, after defeating Frieza. The move is powerful enough to transcend realms as long as its user can lock onto certain ki signatures. The move is used pretty often, but it really should see more use.
Luckily, in the manga, Vegeta is seen training on Yardrat so that probably means he will begin to use the move as well. Up until this point, the only main character who could use the technique was Goku and his fusion forms.

Four Witches Technique

Tien using the Four witches against Goku in Dragon Ball
Also known as the Four Supernatural Fists, is a technique developed by the Triclops people like Tien. Early Dragon Ball and Dragon Ball Z had a lot of race specific techniques that haven't seen the light of day since. This power is one of them. With the Four Witches Technique, the user could grow an extra pair of arms at almost no downside besides time.
Tien's moves almost all seem to have some kind of negative impact like draining his life force or splitting his power up between clones. This is his one technique that doesn't hurt him in any way, and it's only used in one notable occasion in the series.

Special Beam Cannon

Piccolo using his signature beam cannon
Special Beam cannon is to Piccolo the same way Kamehameha is to Goku. The issue with this comparison is that the Kamehameha has gotten tons of use and evolutions throughout the series, as well as use from other characters. Special Beam Cannon has only been used a handful of times in the manga, and other than Piccolo, Gohan is the only other main character who can use it.
In the early days of the series, the Kamehameha needed to be charged over time, just like the Special Beam Cannon, so it's weird to think that only one technique would evolve and grow stronger over time. Special Beam Cannon is one of the most iconic moves of Dragon Ball and deserves to be used more to reflect that.

Masendan

Gohan using the masendan grenade technique
When Gohan trained with Piccolo, he learned moves from the demon clan like the Special Beam Cannon and Masenko. Masenko became his signature move as a kid and he eventually evolved it into his own special form called the Masendan. The Masendan condenses the energy of the Masenko wave into a grenade-like projectile, that explodes on impact.
This move was seen only once in the manga but was a testament to how Gohan had grown as a martial artist, creating his own techniques in the heat of battle and evolving his battle senses. Gohan's uses of demon clan techniques and Turtle Hermit School moves were what made him so unique as a kid before he started to use the same moves as Goku and everyone else.

Great Ape Transformation

image-2021-02-03T091432.345 Toriyama seemed to have just forgotten that Saiyans are born with tails since the Great Ape transformations aren't relevant after DBZ, and GT isn't part of the canon universe. This is a shame because GT showed that being able to master the Great Ape form could lead to wild transformations and huge bursts of power.
The transformation leaves the user in a state of rage like the Hulk, but that doesn't mean its power couldn't be harnessed or mastered. Saiyans relying only on Super Saiyan has become stale which is why forms like Ultra Instinct and Super Saiyan Blue Evolved are starting to pop up. Mastering the Great Ape state would've been another original transformation that naturally progressed Saiyan power levels.

Super Saiyan 3

Goku using SSJ3 Against Buu
Super Saiyan 3 is the most powerful form of Super Saiyan in the manga and anime. Of course, Goku was the first to achieve this form and master it. The form seems to have very limited use since it depletes the ki of the user very quickly. The form isn't suitable for battle when used by Goku but Gotenks was able to get a bit more use out of it.
This transformation is a very iconic one, but it exists mainly to show that it isn't useful. Goku flashes the form to Kefla in the anime as she learns about the Super Saiyan forms, and maybe this is foreshadowing for later since she has some of the highest levels of ki in the universe.

Big Bang Attack

Apart from Galick Gun and Final Flash, Big Bang Attack is probably Vegeta's most recognizable move. What's crazy about this is that he only shouts the move's name a few times in the whole series. Vegeta seems to do it other times like when he blows up the World Tournament Arena, but it's not clear because the colors and sounds are different.
Big Bang Attack is Vegeta's go to Super Saiyan move, and it's a shame that it's only done a few times and is unrecognizable half the time. As he powers up through the story he can unleash it like a regular ki blast, lessening the emotional value and flashiness of the move.

Ultimate Knuckle

The classic Gohan back fist attack
The signature back fist of Gohan has a name, but it isn't well-known by fans. Ultimate Knuckle makes an appearance versus Cell, Goku, and Buu, but it doesn't have any special applications aside from being a back fist. This could be due to his time training with Piccolo who likes to use elbow drops and bashes, but it happens so consistently that it should have more applications.
Melee based moves in Dragon Ball aren't anything uncommon like the Super Dragon Fist, so why not evolve this move further into something more flashy and interesting since it happens so often? It seems to be a product of few characters getting new moves besides Goku, and that problem has been going on for years now.