My thing is Zamasu is using a cheat codes by using Goku's Body and the Super Dragon Balls were as Freeza, Cell and Buu were themselves just mutiple forms. I mean I admit Zamasu/Black is sitting at the table but he clearly is using again cheat codes to be at this level. Now from an intelligence standpoint Yes he's probably the smartest DB Villian next to Cell.
I disagree with you on Cell part. To me, Cell is overrated villain because he wasn't no damn himself to being with because Cell was created by Dr. Gero. If it wasn't for Dr.Gero so Cell wouldn't be exist and if wasn't for Vegeta for his selfishness and pride he could have beaten Cell. That how he was the reason Cell reached the perfect cell.
The closest villain that matched Frieza is Kid Buu (seriously, the guy is pure evil), but even then, I'd still give the edge to Frieza because his destruction was planned and systematic; Kid Buu just wanted to f*ck up everything in his path. Kidd Buu is just PURE EVIL,& RETARDED because he was actually original to being with. Frieza is a greatest villian. Frieza was like Hitler that hating on saiyan like jew . Cell was a computer program.
Also..You miss the part when Beerus woke up, he actually mentioned about Frieza and Kidd Buu once Whis explained to him what Goku did to them and Beerus didn't mention Cell because he didn't exist as an original. He was created by Dr.Gero.