Your choice, but just to offer a shortlist:
Broly
Babidi
Android 19 and Dr. Gero
Dodoria
Zarbon
Ginyu Force (choose individual or the whole team)
Captain Ginyu
Vegeta and Nappa with Saibamen
Cell with Cell Juniors
Frieza
Mecha Frieza and King Cold
Cooler and Armoured Squadron
Least favourite/ worst villain throughout DBZ. You can mention your favourite if you want too.
Slight edit: If you have a least favourite, and also a villain you think was most evil, feel free to mention both. In fact, give a rundown of as many villains as you like, just saying what you made of them in general, who stood out, good or bad, and why. Apologies for any confusion, I should have made it more specific, but as with most of my blogs, you're free to write as much as you like, and take the question liberally, which is to say there's no one way to approach it, you can take it in your own direction if you want :)