My favorite is by far the Bardock Special (excluding, of course, the Jump special). It has a very interesting backstory to the Freeza saga, and honestly, the visuals and the story, to me, anyway, is reminiscent of Star Trek.
The trunks special is cool, too, but honestly, even though it is a dark story and such, it's not as dark as the actual android saga alludes to.
"That's Peter Grave's Scat! I'd recognize it anywhere!" - The Film Crew