Like it says on the tin.
Knowing what we know of Gidorah's three heads, its most likely that it was Ni(the fighting-fanatic head of Gidorah) that ended up in the Mechagodzilla body, going by how it was acting: both savagely, and sadistically.
If it had been Ichi, chances are that we would have seen a Mecha-G which was strategizing and fighting smart, while also incorporating all the brute force his new chassis could offer him. IMO this Mecha-G would have been much more efficient and intelligent. It would also likely have been the winner.
BUT.
What if it had been San, or Kevin, Gidorah's STUPID HEAD which took over the Mechagodzilla body?