GT Bios- Baby Vegeta
This is the parasite Baby, after he successfully entered Vegeta's body and took over his sanity. He is one of the major supervillains in the series and is yet the most powerful villain Goku would have to overcome. This is Baby Vegeta's weakest form, but he has two more transformations at his disposal that can take his already immeasurable power to a whole new level.
When Goku met General Rilldo, he was amazed he stumbled across a being whose kii was even greater than Majin Buu's(Goku was most likely referring to Kid Buu). It is implied in Dragon Ball GT that even Rilldo's base form is stronger than Kid Buu and the previous Super Saiyan 3 Goku. When Goku and Rilldo fought each other at their base, Rilldo was outclassed and it became clear that Goku could effortlessly defeat him, meaning that Kid Goku at base is much stronger than his Z Super Saiyan 3 counterpart. When Goku transformed into a Super Saiyan, Rilldo commented that his power was multiplied by a hundred fold(that's over one-hundred times greater than Rilldo's and Kid Buu's). Rilldo realized that there was no way he could match Super Saiyan Goku and was forced to transform into his second form. Regardless, Goku still proved to be too strong for him.
Later, when Goku met Baby Vegeta, he was even more amazed than he was during his meeting with Rilldo. Goku never thought that merging with Vegeta would give Baby the kind of power that is beyond what reality can grant. Baby Vegeta easily defeated Goku in his Super Saiyan 3 form.
Note: Rilldo's second form just might be stronger than any form of Majin Buu, though it goes unnoticed. Also, considering the fact that Baby in this form can effortlessly defeat kid Goku in his Super Saiyan 3, proves that Baby Vegeta is in a completely different league.
Aside from the Super Saiyan 4 transformation, this is one of Vegeta's strongest forms. Baby's power in this form is unprecedented, as shown when a punch to his face by Super Saiyan 3 Goku did very little. This form of Baby is only seen briefly in the series.