GT power scaling is pretty terrible. I'll go with Vegito.
I say SSj4 Goku, people criticise GT and rightly so for it's inconsistencies and lack of powerful showings but simple DBZ logic still dictates that Goku over a decade after the end of Z, having mastered a new more efficient form would destroy ssj3 Goku or anyone else from Z. Just because the execution isn't great does not mean the idea and character profile doesn't remain the same. SSj4 Goku was obviously intended to be vastly more powerful than any of the characters in Z otherwise there is no point in the transformation.
SSj1 Vegitto would present a challenge but given over a decade of extra training there is a very good chance Goku could have quadrupled his strength, just look at what he accomplished in the roughly 10 year period between the androids and Buu. He has an insane sparring partner in Uub, other guys like Veget and Piccollo who are training beasts, unlimted resources through his connections at Capsule Corp and can Instant Transmit anywhere in the universe or otherworld that will benefit his training.
If this is ssj4 Goku returned after departing on Shenron at the end of GT I think he could take even full-powerd Vegetto without too much difficulty.
By feats alone Vegito wins easily in fact the feats of early DBZ >>>> later DBZ>>>>>>>>>>>DBGT, however if you use power scaling then ssj4 Goku wins easily, he himself stated that fusion was useless against Baby when Goten and Trunks were about to fuse, this suggesting that Baby > Buu by a great deal.
I gotta ask something, alot of people are saying that an SSJ2 is x2 stronger do people mean an SSJ2 gives only an x2 boost or do they mean an SSJ2 is x2 stronger than that of an SSJ meaning an SSJ2 gives an x100 boost in power? Lets say these PL's:
Goku SSJ - 2000
Goku SSJ2 - 4000 (x2)
Goku SSJ2 - 200000 (x100)
If i use the SSJ2=x2 and SSJ3=x4 then are these PL's examples correct?
Gotenks SSJ - 2000
Kid Goku (base) - 2000
Gotenks SSJ2 - 4000 (x2)
Gotenks SSJ3 - 16.000 (x4)
Kid Goku SSJ - 100.000 (x50)
Kid Goku SSJ2 - 200.000 (x2)
Kid Goku SSJ3 - 800.000 (x4)
Forgive me if i'm totally wrong at this or these calculations because math isn't my strongest point :) If SSJ2 only gives x2 like above then it doesn't look good for Vegito. We also don't know how much Golden Oozaru and SSJ4 gives. Goku at SSJ3 could't even harm Super Baby Vegeta he laughed at everything Goku trew at him. He then goes Golden Oozaru and is strong enough to put the hurt on Baby but Super Baby also manged to hurt the giant ape. Goku then transforms into SSJ4 and proceeds to tank Super Baby's most powerfull attacks (Revenge Death Ball) and doesn't even flinch hell Goku let Baby pound on him for a couple of minutes to see how strong he became.
SSJ = 50x base (well its supposedly is anyway)
SSJ2 = 100x base or 2x SSJ
SSJ3 = 400x base or 4x SSJ2
SSJ4 is probably 500x considering its simply a concentrated version of Golden Great Ape ( SSJ 50 x Great Ape 10)
anyway i found this if anyone wants to see it
no but your getting the wrong idea super vegito hardly made a effort to dominate super buu (gohan absorbed) and also with the baby saga goku's body wasn't strong enough to cope with the enormous power of a super saiyan 3 which I don't blame since shenron wished for goku to be a child and so therefore he wasn't strong enough to defeat baby
so I think super vegito would win because of the fact that he can turn into a super saiyan 1 , 2 ,3 ,4 if he wanted but he didn't because it was a waste of energy. and vegito can easily dominate ss3 goku and therefore he can dominate super saiyan 4 goku if he went ss2 but you can't judge on how much power vegito used in the series you should judge him in a way that he has a lot of power and he too can go ss3 like gotenks did
so I would say vegito wins this one
Power scalling will lead nowhere. There will never be a fusion formula usable in battle forums that will give you one clear number. So far the best interpretation is done by Infamous_666 in here http://www.neoseeker.com/forums/88/t1655328-metamorian-potara-fusion/ and nobody can top that, trust I've done more research on than all of you, this is the simplest one that doesn't result in holes, and it's useless due to the unknown E factor.
Here is my estimate in nutshell:
Base goku 1000
Goku was on SSJ3 level at the start of the GT in base form.
I think Goku and Vegeta is on same level, so the fusion:
So Vegetto win in SSJ3, but not in SSJ2.
Peaple say that Goku SSJ3 and Gotenks SSJ are in the same league so if we use that:
Kid Goku PL - 5000
Gotenks SSJ PL - 5000
Kid Goku SSJ PL - 250.000
Gotenks SSJ2 PL - 500.000
Kid Goku SSJ2 PL - 25.000.000
Gotenks SSJ3 PL - 200.000.000
Kid Goku SSJ3 PL - 10.000.000.000
Ultimate Gohan is probably between Gotenks SSJ3 and Kid Goku SSJ3 or perhaps is equal with Kid Goku SSJ3 who is 4x stronger than Gotenks SSJ3 witch is alot.
When Kid Goku SSJ3 fought Baby Vegeta in his base form he was the better man so Baby Vegeta>Kid Goku SSJ3. Baby Vegeta then recieved the powers from Gohan/Goten/Trunks and Bula witch should have increased his powers substantially (keep in mind Gohan probably is weaker in GT and none of them where SSJ during the boost, if that matters at all). He then also recieved a power boost from all the humans on Planet Tuffle, now it might sound like nothing but remember when Goku gathered energy for the Spirit Bomb that destroyed Kid Buu? the ball looked pretty big with all the Z-Fighters+Planet Namek's energy already gathered but, when earth's energy came the ball grew to nearly 2x the size so the energy gain from the humans should not be taken lightly.
Now Super Baby Vegeta is WAY beyond a mere Kid Goku SSJ3 considering all the buffs he recieved and some people actually pit Super Baby Vegeta and Buutenks/Mystic Buu against eachother. Golden Oozaru was more than a match for Super Baby Vegeta so is Super Baby is PROBABLY a match for the 2 Super buu's than the Golden Oozaru should be a 100% match. Super baby wasn't a total loss as he still manged to hurt the Ape by a litte but in the end Super baby Vegeta was outclassed.
Goku SSJ4 then appears and showed way more power than the Great Ape, letting Super Baby Vegeta hit him for the lulz and actually tanking the full force of The Revenge Death Ball like it was nothing. Goku SSJ4 should be alot stronger than Vegeto in his base. Vegito however has still his SSJ transformation increasing his power by x50 and its possible that Super Vegito still be stronger than Goku SSJ4. However Goku SSJ4 duing the Shadow Dragon Saga would pose a problem because by then he became stronger and if we include the most pwoerfull incarnation than i'm afraid SUper Vegito doesn't stand a chance because Goku SSJ4 recieved a similair boost than Super Baby Vegeta. Gohan/Trunks/Goten all SSJ were giving it their all to power up Goku even to the point that Goku could't almost handle the power.
But ofcourse these fan made powerlevels of mine are mere speculation because....we have no official info do we :) I say Ultimate Gohan might be just as strong as Kid Goku SSJ3 but for all i know Gohan is 40x stonger than that. People say that Kid Goku is just as strong as SSJ3 Goku from DBZ because Rildo supposedly was just as strong as Majin Buu. Notice the word MAJIN BUU, not Kid Buu/Fat Buu/Evil Buu for all we know he means Mystic Buu, even during the fight the powerscaling was off. Goku might have sensed how strong Rildo was but Goku didn't knew Rildo would fuse with The Sigma Force witch would have increased his power, yet kid Goku lolstomped Sigma Force Rildo, Goku only went SSJ against Metal Rildo.
If Kid Goku is just as strong as Kid Buu/Goku SSJ3 (DBZ) than i think there could be a debate on who might win but for all we know he ment Mystic Buu, then Vegito really doesn't stand a chance. likewise we also don't know the Golden Oozaru and SSJ4 multiplier for all we know the Golden Oozaru is x500 and SSJ4 is x600 but it might be even less than that. Speculations speculations :)