Raditz: Didn't do shyt
Nappa: Killed some of the Z warriors but got beat down and obliterated by Vegeta
Vegeta: Strong as hell back then, took the remaining Z warriors left alive to stop him. After that tho he pretty much became a good guy. Besides, he had too many losses to be considered BEST villain.
Prince of Sayians, but King of L's.
One of Frieza's minions (Zarbon, Ginyu Force, etc.): fukk em
Frieza: destroyed planet vegeta, wreaked havoc on Namek, made Vegeta cry, made Goku go SSJ by killing Krillen. Dude has a damn good resume.
The Androids: Didn't really do shyt
Cell: forced Gohan into going SSJ2, responsible for Goku and Trunks' death, abosorbed and killed a bunch of people before he became perfect.
Dabura: fukk him
Buu: Out of all the villains Buu may be the most entertaining. His interactions with Gotenks, Piccolo and Mr. Satan were priceless. Obviously he was strong as hell and even managed to destroy Earth.
When I think of best villain I look at the villains impact on the main characters, impact on the plot, how successful they were, and their personalities. So I would rank them as:
1) Frieza
2) Buu
3) Cell
the rest don't matter. Vegeta spent more time as a grey character than an actual villain.
Freiza had the total package, Buu had the best personality but I don't consider his impacts as significant as Freiza, and Cell was ok but falls short of the others.
If all of Dragonball was included I'd put King Piccolo at 3, maybe 2.