So, if I were to answer this question I'm going to lean towards the main fight in Dragonball Movie #6 'Return of Cooler'. Mainly, because I believe it's the biggest tribute to the series at the time of it's creation. Goku is Goku, Vegeta is actually Vegeta and they battle an interesting nemesis in Meta Cooler who regenerates through some sort of mechanical method at the behest of some sort of Death Star twist. I'd say he's probably the most insane concept of any enemy I've ever seen. I don't want to spoil the movie by explaining, but I recommend watching it ASAP.
What's yours?