Saiyan Saga

Radditz: 1 200
Farmer: 5
Goku with weighted clothing: 334
Goku w/o weighted clothing: 416
Piccolo with weighted clothing: 322
Piccolo w/o weighted clothing: 408
Goku Kamehameha: 924
Piccolo Makankosappa: 1 330
Piccolo 2nd Makankosappo: 1 440

Gohan: Varies from 1 to 1 307 All power levels are stated from Radditz's scouter.

Nappa & Saibamen

Saibamen: 1 200
Piccolo: 1 440
Tienshinhan: 1 330
Krillin: 1 180
Yamcha: 1 220
Chaozu: 1 020
Gohan: 980
Nappa: 4 000 The Z fighters beat the Saibamen but end up losing Yamcha. When they fight Nappa, Tien, Chaozu and Piccolo were killed. The readings might not be as accurate because the Z fighters know how to lower their fighting power.

Vegeta

Vegeta: 18 000
Goku: 5 000-8 000
Goku Kaioken: 12 000
Kaioken x2: 16 000
Kaioken x3: 24 000
Kaioken x4: 32 000
Oozaru Vegeta: 180 000 Goku does pretty good using his kaioken technique, until Vegeta goes oozaru and knocks him around. Kaioken, when used, multiplies the power level by whatever times the kaioken.
Eg. 8 000 x 4 = 32 000

Freeza Saga

Beginning of Freeza saga

Gohan & Krillen: 4 000 Each
Namek Fighters: 3 000
Dodoria: 20 000
Zarbon: 22 000 Dodoria literally pounds on the Namek fighters, not much of a fight. Gohan and Krillen comes in and saves Dende from Dodoria.

Vegeta VS Dodoria & Zarbon

Kui: 18 000
Dodoria: 20 000
Zarbon: 22 000
Vegeta: 24 000
Zarbon Monster: 32 000
Vegeta Healed: 40 000 Vegeta wastes Kui and Dodoria, he is able to beat Zarbon but when Zarbon transform he lays the smackdown on Vegeta pretty hardcore. After Vegeta is healed in the rejuv tank he makes monster mash out of Zarbon Monster.

Ginyu Force

Krillen & Gohan: 15 000
Vegeta: 40 000
Guldo: 10 000
Recoome: 55 000 Guldo gets wasted by Vegeta, while Recoome wastes Gohan, Krillen and Vegeta.

Goku VS Ginyu Force

Jeice: 45 000
Burter: 50 000
Recoome: 55 000
Captain Ginyu: 120 000
Goku hidden's PL: 5 000
Goku Fighting: 60 000-180 000
Ginyu in Goku's body: 23 000
Krillen & Gohan after senzu beans: 25 000
Vegeta after senzu bean: 100 000 Goku kicks the Ginyu Force's butt with little or no trouble. Vegeta gets a big power up with the senzu bean, beats Jeice and Ginyu in Goku's body.

Time for Freeza

Nail: 42 000
Vegeta: 200 000
Freeza form 1: 530 000
Piccolo merged with Nail: 750 000
Freeza form 2: 1 000 000
Freeza form 3: 2 500 000
Vegeta after Dende healing: 1 000 000 Although it might not look like it, but Vegeta actually puts up a fight against Freeza form 1. When Piccolo comes, although weaker, beats Freeza form 2 forcing him to change to form 3. After Vegeta gets healing from dende he thinks he can take on Freeza, but Freeza is only using a fraction of his of his full power.

Goku VS Freeza

Goku: 300 000
Goku kaioken x10: 3 000 000
Goku kaioken x20: 6 000 000
Freeza using 50% power: 6 000 000
Freeza using 75%: 8 500 000
Freeza using 100%: 12 000 000
Super Saiyan Goku: 15 000 000 Goku is getting whooped by Freeza, after Goku lands the Genki-Dama, Freeza kills Krillen. This causes Goku to go Super Saiyan and begins to whip the floor with Freeza.

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There are no more 'official' power levels after this point, I have yet to see an episode where they state the fighters 'ki', or where they use scouters. Thus, every power level after this is a complete guess, so please try to hold off those 'It's all wrong' emails.
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King Cold

Trunks' hidden PL: 5
Freeza Bot: 15 000 000
King Cold: 20 000 000
Super Saiyan Trunks: 25 000 000 Not much to say here, Trunks slice n' dices Freeza Bot to nothingness, kills King Cold and destroys their ship.

Cell Saga

Arrival of Androids

SSJ Trunks: 35 000 000
SSJ Vegeta: 40 000 000
SSJ Goku: 45 000 000
Sick Goku: 10 000 000
Android 16: 55 000 000
Android 17 & 18: Both 50 000 000
Android 19: 15 000 000
Android 20: 7 500 000
Piccolo merged w/ Kami: 50 000 000
Cell form 1: 45 000 000
Cell after absorbing humans: 55 000 000
Cell after absorbing Android 17: 75 000 000
Perfect Cell: 100 000 000 The Z fighters were doing well, they beat up Android 19 & 20 pretty easily, everyone gets stronger in the S & T room. Cell arrives, after Cell absorbs 17 & 18 he becomes perfect and decides to have a tournament, 'The Cell Game'

Cell Game

Mr. Satan: 120
SSJ Trunks: 95 000 000
SSJ Vegeta: 95 000 000
SSJ Goku: 115 000 000
Perfect Cell: 115 000 000
Ultra Perfect Cell: 150 000 000
Cell Juniors: 100 000 000
SSJ Gohan: 75 000 000
SSJ2 Gohan: 150 000 000 Let me sum things up for you. Goku fights Cell, Goku loses and says that Gohan will fight next. Gohan is losing until Cell spits out 7 Cell juniors to beat up the other Z fighter. Gohan goes Super Saiyan 2, Gohan beats Cell, Cell self-destructs killing Goku. Cell comes back Ultra Perfect. Gohan and Cell fire Kamehamehas, Gohan wins. End of story.

Buu Saga

Before Arrival Of Buu

SSJ2 Gohan: 165 000 000
SSJ Vegeta: 185 000 000
SSJ Goku: 200 000 000
Chibi Trunks: 1 900 000
SSJ Trunks: 95 000 000
Goten: 1 800 000
SSJ Goten: 90 000 000
Piccolo: 125 000 000 This is just before Buu is revived the Z fighters are participating in the 25th Tenkaichi Budoukai.

After Arrival Of Buu

Dabura: 175 000 000
Fat Buu: 250 000 000
Skinny Buu: 275 000 000
Super Buu: 315 000 000
Majin Vegeta: 225 000 000
SSJ2 Goku: 225 000 000
SSJ3 Goku: 300 000 000 With the addition of Majin Vegeta's power, Buu is now revived. Buu easily defeats Dabura and Gohan. Then he starts to wreck havoc across the Earth.

Majin Buu

Gotenks: 175 000 000
SSJ Gotenks: 250 000 000
SSJ3 Gotenks: 325 000 000
Gohan Zed Sword Training: 250 000 000
Chou Gohan: 350 000 000
Majin Buu: 300 000 000
w/ Gotenks & Piccolo: 325 000 000
w/ Goten, Trunks and Piccolo: 375 000 000
w/ above plus Gohan: 400 000 000
Vegetto: 425 000 000
SSJ2 Vegetto: 500 000 000
Kid Buu: 300 000 000 Majin Buu is kicking everyone's asses, when he absorbs Gotenks, Piccolo and Gohan he gets even stronger. Then comes Goku and Vegeta to save the day, they fusion to become Vegetto and smoke Majin Buu.

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Because there were no previous power levels stated in Dragonball GT, it is difficult to give them an accurate reading. However, I assume that the power levels would be higher. In Dragonball GT Bebi-Vegeta had no problem disposing of Ubuu fused with Fat Buu, Ubuu was the reincarnation of Majin Buu and Majin Buu was the strongest villian in Dragonball Z. Note: All SSJ levels I wirte here are SSJ at full power, that includes SSJ2 and/or 3, unless otherwise stated.
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Bebi Saga

Black Star Dragonballs

Goku: 7 500 000
Ubuu: 7 000 000
SSJ Goku: 350 000 000
Rirudo: 300 000 000
Pan: 1 500 000
SSJ Trunks: 175 000 000
Bebi: 20 000 000 Goku, Pan & Trunks go into space searching for the Black Star Dragonballs. On thier journey they encounter minor villains, until they land on one planet inhabited by Bebi.

Bebi-Vegeta

SSJ Goten: 175 000 000
SSJ2 Gohan: 275 000 000
SSJ2 Vegeta: 325 000 000
Bebi: 25 000 000
w/ Trunks & Goten's Power: 250 000 000
w/ Above + Gohan's Power: 400 000 000
Bebi-Vegeta: 550 000 000 Bebi manages to sneak onto Earth, after taking over Goten and then Gohan, Bebi uses his new power to overcome Vegeta. He beats Vegeta and uses Vegeta's body as a host for him.

Bebi-Vegeta Vs Goku

Bebi-Vegeta: 550 000 000
Ubuu powered up: 350 000 000
Mr. Buu: 325 000 000
Ubuu-Buu: 525 000 000
SSJ3 Goku: 500 000 000 Even with the combined efforts of Ubuu and Buu, they are no match for Bebi-Vegeta.

Super Saiyan 4 Goku

Gold Oozaru Goku: 600 000 000
SSJ4 Goku: 650 000 000
Gold Oozaru Bebi: 625 000 000 SSJ4 Goku faces off against Bebi-Vegeta. After a lengthy battle Goku prevails and Bebi leaves Vegeta's body only to be killed by Goku's Kamehameha.

Super 17 Saga

Super 17

SSJ2 Gohan: 400 000 000
SSJ Trunks: 250 000 000
SSJ Goten: 250 000 000
SSJ2 Vegeta: 550 000 000
Ubuu: 525 000 000
Cell: 250 000 000
Freeza: 200 000 000
Super 17: 650 000 000
SSJ4 Goku: 650 000 000 Super 17 beats up the Saiyans quite easily, he is winning against Goku when Android 18 jumps in. She distracts him long enough for Goku to pull off the Dragon Fist and destroy him.

7 Evil Shenlons

Evil Shenlons

6 Shenlons: 50 000 000-100 000 000
Li-Shenlon: 750 000 000
SSJ4 Goku: 700 000 000
SSJ4 Vegeta: 650 000 000
SSJ4 Gogeta: 900 000 000
Goku Genki-Dama: 1 000 000 000 There is no trouble disposing of the 6 Shenlon's, until the strongest Li-Shenlon appears. Gogeta is formed and beats up Li-Shenlon, but the fusion wears off before they can kill him. Goku now begins to power up the strongest Genki-Dama in Dragonball history.