My take:
Freiza > Saiyan > Cell > Buu
But hot darn, it's memorable all over the place.
And somehow, Dragon Ball's closing theme is as nostalgic to me as anything else.
Buu last is crazy to me, but I haven't seriously watched DBZ since the mid 00s.
Majin Buu and all his forms and their different emotions were super cool / scary.
We got Majin Vegeta from this. We first see SS3.
Goten and Trunks fuse to make Gotenks... Super crazy
Gohan fights Super Buu who's consumed Gotenks / Piccolo etc and just running wild
Goku and Vegeta fusing into Vegito
They go INSIDE Super Buu to find all of the people he's consumed and are travelling through his body while Buu is getting furious and looking inside himself at them
And then it takes a Spirit Bomb to destroy Kid Buu
My recollection of Cell is rusty. Cell Games was amazing.
Frieza saga 10/10
Saiyan saga was awesome too.
Idk impossible to rank for me.
Other shounen have surpassed DBZ for me in terms of how much I enjoyed them, but it was unquestionably the God Father and the gateway into a whole new world for a lot of people.