Buu was a very powerful opponent in the DBZ universe and many would argue he should be higher than 5, but I just can’t get past him turning people into candy and basically being a 5 year old. Yes he does become more intense as he transforms but it’s not enough to get past his childish persona. In my eyes, that hurts him as a villain.
The legendary Saiyen (or its Goku depend on how you look at it). Broly always seems to come back with one thing on his mind “Kakarot”. His power level is off the chart as a baby and gets to the point where he is almost too much for Goku Gohan and Goten to handle.
Fun fact, Frieza is easily the most powerful villain in the DBZ universe. The reason he loses to Goku is because he never trains and just forces himself to what was thought to be his final form. We later learn in Resurrection F that was not it. Also we learn that Golden Freeza is strong enough to beat a Super Saiyan God, but he doesn’t train enough to be able to maintain that form without losing all of his energy. Had he trained his body more, he wins that fight.
Honestly, Vegeta should be #1, he’s such a badass, clearly reaches levels that no villain is able to reach (sans Golden Freeza [kind of]). He is either your first or second favorite character in the show. He pushes Goku helps push Goku to limits never thought possible. Hes also the only person allowed to call Goku by his real name. In the Buu Saga he returns to an evil form and has an amazing fight with Goku, that however brings me to my next point. Vegeta can not be #1 because he is more of a good guy throughout the series, even if 90% of his actions are to benefit him.
Cell is the #1 villain in all of Dragon Ball Z. Already knowing everyone’s moves and then his cells are able to absorb the ability learn more of his opponents moves. He blows himself up and is still able to live. He’s the only fighter to actually kill Goku, and even before killing him (via self destruct) Goku would not have beaten Cell (Goku admits this before Gohan steps in). Also Cell literally shit talks while kicking ass, dudes the man.