Once again, I am going completely cliché with this one. When Goku went Super Saiyan against Freiza.
To me, that is the high point of Dragon Ball Z. Ever since the beginning of the Dragon Ball, the entire series was built up to this point. This also must do with the stakes of the situation as well. After fighting an extended losing battle to Freiza, because Freiza has an infinite number of transformation sequences. There was a last desperate attempt to take down Freiza in the form of a spirit bomb, but that didn’t work. When Goku and crew were celebrating, Freiza came up and attempted to murder Piccolo and then actually murdered Krillin by crushing him. After all the years that they were friends, Krillin was gone. Through the pain of loss, Goku pushed through the barrier and finally made it to Super Saiyan. This was a moment that I couldn’t wait for as a kid. It’s embedded into my mind deeply and it truly was the best that Dragon Ball Z had to offer.
Ever since that moment, Super Saiyan and Dragon Ball Z never felt the same. There was absolutely nothing special about it. Other saiyans and half saiyans could do it too. It became a “so what” kind of deal. Gohan powering up to Super Saiyan two and Goku powering up to Super Saiyan Three tried to replicate the hype and excitement of this scene, but they just couldn’t.