The legendary Mel Blanc provided the voice for Cosmo in the original series. His Spacely is just another iconic voice in a resume filled with iconic voices.
Frank Welker provided the voices for both the child and teenage version of Cosmo in the Christmas episode. He gives a good raspiness to the child version, which helps sell the idea that his voice will eventually turn into the more gravelly 'loud-and-angry' voice of Mel's. The teenage voice is lighter, similar to his Fred Jones performance. And, unfortunately, it loses much of its uniqueness because of that. It turns into a generic 'young adult male' voice.
Due to Blanc's unfortunate passing in the middle of the film's production, Jeff Bergman took over the role (as he did with George, as well). Jeff does a pretty good job of imitating Mel (most of his career has been doing that), but doesn't quite capture the same charm as him.