Probably Vegeta and Frieza given the ridiculous amount of power-ups, exterior help and some asspulls like Gohan's tail suddently growing back or Frieza taking so long to notice the Genkidama above him for them to be defeated.
Vegeta = 1 powerup to beat - KK
Frieza = 1 major powerup to beat - SSJ
Buu = SSJ2 for all the main cast, SSJ3 for Goku, Goten and Trunks learning fusion and SSJ/3, Goku and Vegeta Potara fusion
HMMMMMM
Picolo - Weaker than Goku, only issue was killing him meant no Dragon Balls.
Vegeta - Weaker than Goku with his KK abilities, plus had a few tough allies left to help.
Freeza - Had to unlock a whole new form to beat, and even then he almost lost.
Cell - Vegeta/Gohan/Trunks/Goku probably could've made an effective team if they fought together, and Gohan was a ticking time bomb anyways.
Buu - Goku could've beat him. Gohan was on the cusp of beating him.
Frieza's it for me.
Goku was nowhere near - he'd have needed at least one powerup to even compete
Gohan, again, could have won, but Buu has more difficult powers to fight against - being infinite regeneration (to a greater extent than Cell) and absorption
Id say vegeta or frieza
Piccolo was weaker than goku
Yeah but dragonballs
Vegeta could have killed cell
Nope. No he couldnt. He could barely defeat one single Cell Jr.
Gohan could have killed cell
Only after a powerup that literally DOUBLED his power
Ssj3 could have probably taken the fat buu
No it couldnt - Buu's regeneration is infinite and SSJ3 has a severe time limit
Vegito could have killed gohan buu
Ultimate gohan would have destroyed kodd buu
Ultimate Gohan couldn't destroy him because Buu would just regenerate. He was unable to wield any techs that could vaporise him out of existence.
You cant brush off anything with Vegito. Vegito was literally KAIO's last resort. GODS couldn't handle Buu. That's LEVELS above any other enemy in terms of difficulty.