#1 - Bulma
#2 - Dr. Gero
#3 - Vegeta
#4 - Freeza
#5 - Piccolo
#6 - Kuririn
#7 - Gohan
#8 - Videl
#9 - Cell
#10 - Dr. Briefs
Bulma was capable of creating a Time Machine, has been described as a genuis by the likes Senbei and her father and has been such since an early age, so her scientific knowledge places her intelligence above the rest. Gero is next for similar expertise. Vegeta is 3rd for his intelligent tactics on Namek, his escape from Freeza's spaceship being a stand out and immediately analysing what kind of technique the Kienzan was, whilst Freeza comes next due to catching on quickly to things such as Moori aiming for the Scouters or Dende's healing ability and countering such accordingly. Piccolo is next as whilst he's attempted to be portrayed as one of the smarter fighters, he doesn't make many smart observations outside of combat aside from noting what a good choice in terrain for the battle Gero made. Kuririn is next for the exact opposite, making quite a few intelligent decisions and a few noteworthy tactics, yet failing time after time to make good use of the Kienzan + Taiyoken combo. Gohan is 7th due to generally being a well read scholar and being more rational than most of the other cast members, whilst his wife is next due to her figuring out the Great Saiyaman's identity being a well played move. Cell is next due to some good hit and run tactics in his first form and quick observations in battle, yet is no higher due to making some dumb decisions such as forgetting about Piccolo's regeneration. Finally, there's Dr. Briefs as whilst he may be a scientific genius, his intellect in other areas is little, such as mistaking the police officer for Goku when they first met.
There are some older, sagely characters I could've included like Roshi, Karin, Kami, Kaio, etc. but didn't due to them relying purely on their reputation, being more hype than feats. I could've also included Dr. Mu for similar reasons as Gero, but didn't seeing as how he was a puppet for Baby, so the creation of the Machine Mutants and their power was at best a 50 - 50 split.