sacrificing himself to save Gohan from Nappa. It's the ultimate culmination of Piccolo's character arc, and it's that much more effective if you watched the original Dragon Ball before Z. I know the subtitles don't necessarily match (that's because the subs are for the Japanese language track), but they point out a particularly important point: Piccolo is the Great Demon King
. It's just. Man. Then this music
accompanied that scene in the original. This leads to a genuinely poignant moment after the Saiyan fight, when Gohan defies his mother and declares that he's going with Bulma and Krillin to Namek, specifically for Piccolo. And then even in Dragon Ball GT (which is generally forgettable) has another genuinely poignant moment, when Piccolo (in order to seal the dangerous Black Star Dragon Balls, which were created when Kami and Piccolo were originally one) elects to stay on the decaying Earth and die with the planet, which is the side-effect of making a wish on said Black Star Dragon Balls.
Of course, Goku's sacrifice during the Cell Games is pretty heart-wrenching as well. Definitely the culmination of his character arc, after his unfair treatment of Gohan during that arc.
And I'm surprised nobody's mentioned Vegeta's sacrifice against Majin Buu. Yet again, the culmination of his character arc. He even knew his soul would be headed straight for Hell, and did it anyways, in order to protect his wife, his son, and his friends. Too bad the latter half of the Buu arc is such a mess, narratively speaking.