Include a Youtube link if you want, but name it, and if you can remember, name the scene that best aligns with it. Mine is SSJ3 Goku, when he did it for the first time, http://www.youtube.com/watch?v=dYDohBeP72M (that's also one of the best clips in Dragonball Z, I love the way he explains the transformations, and the animation is beautiful!).
But for a fairly long time, until I saw that episode in the Buu saga, it was Vegeta in the rain, after 18 humiliated him http://www.youtube.com/watch?v=NELtVAaw9ac (this is the clip in full)
http://www.youtube.com/watch?v=mrogZTkvRxs (this is the specific song)
And don't feel limited to the FUNimation dub, just give your favourite music track/ theme, if it's Ocean or Kai or the Japanese then please say so