Yu Yu Hakusho's most iconic weapon has to be Yusuke Urameshi's Spirit Gun. It's been a staple since the series' beginning, and he's repeatedly improved and enhanced the technique over time.
Rather than trade it in for a more powerful move, Yusuke heavily relies on it and develops it until the very end -- and he has his "pacifier junkie" boss Koenma to thank for it. Here's how the Spirit Detective made his finger into Yu Yu Hakusho's number one attack.
Yusuke first learned the Spirit Gun after returning to life at the very beginning of Yu Yu Hakusho. He's been framed for theft by a corrupt teacher, but Koenma needs him to go after three dangerous demons who have stolen three very important artifacts from the Spirit World vault. Yusuke, however, wants to get revenge on the teacher for trying to get him kicked out of school on his second day back. So Koenma gives him a quick lesson on how to channel his spirit energy into the tip of his finger so nobody will see Yusuke hurting a teacher. However, he can only use this power once a day at first.
After Yusuke fights Gouki for the Orb of Baast and promptly gets his butt handed to him, Botan gives him the Concentration Ring to give his Spirit Gun a little boost. However, using it will completely deplete his energy and leave him vulnerable, so she tells him to only use it if he absolutely has to. Yusuke then strengthens his Spirit Gun enough to defeat Gouki by shooting it down his throat. During his fight with Hiei, Yusuke -- and pretty much everyone else -- learned that spirit energy can bounce off mirrors when he uses the Forlorn Hope artifact to reflect his Spirit Gun and shoot Hiei from the back.
It wasn't until meeting Genkai that Yusuke learned to use it more than once a day -- and with better efficiency. After his intensive training, he's able to fire multiple shots at once, calling it his Shotgun. When firing this version, he channels his spirit energy into his fist rather than his finger and takes out multiple opponents at one time. He has a movie-exclusive alternate version of this attack called the Shotgun Double Barrel, where he fires a Shotgun from both of his fists rather than just one.
During the Dark Tournament's Final Round, Yusuke channels all of his remaining energy into a large ball, firing it at Younger Toguro. While not named in the manga or anime, the Yu Yu Hakusho games call this the Spirit Gun Mega attack. He also develops an attack where he rapid-fires two shots of Spirit Energy. The ultimate version of these attacks comes after his demon side is revealed during his fight with Shinobu Sensui.
After Raizen awakened Yusuke's inner Mazoku blood, the protagonist could then channel this newfound Demon Energy into his Spirit Gun and fire it in the same way. He uses these new attacks during his fight with Sensui in Demon World, launching a Demon Gun Mega at the former Spirit Detective. Yusuke continues developing his energy manipulation abilities to fire his own unique version of Sacred Energy.
There is an attack exclusive to the second movie, Yu Yu Hakusho: Poltergeist Report, where Yusuke uses the Power Sphere to manipulate his Spirit Gun in a way that reflects his friends' signature attacks. The bullet fires normally then slashes toward his opponent like Kurama's Rose Whip. Yusuke then stabs at his opponent like Kuwabara would with his Spirit Sword, before finally engulfing the target while combining with Hiei's Dragon of the Darkness Flame. This attack is done in tandem with everyone else influencing the bullet's movement.