Possibly first form (post-training) if he truly is the most powerful opponent the Z-senshi had ever felt up until that point. Though maybe that's getting a little carried away.
No one, obviously; even SSG isn't enough to beat him. That said, when it comes to 4th Form Frieza, I'm going to hold my peace until I see his manga performance in the TOP.
Nah, you're right. First Form Frieza could potentially be superior to even the likes of Super Buu and Buuhan.
The only person from DBZ that could be argued as post-training first form Frieza's equal is Super Vegetto himself, and that's only because he was clearly above Buuhan and no one from the Z-senshi had ever felt Vegetto's power because they were all either dead or unconscious for the hour that he existed.
First form Frieza could be perceived as around a Buuhan level character, going by statements. Considering that their wasn't much difference between Buutenks and Buuhan, and Gohan himself had felt Buutenks' power, that should put First form Frieza above Buutenks, and thusly around the level of Buuhan.
Fourth form should effectively one-shot anyone from DBZ, including Super Vegetto. Even if Vegetto could go SSJ2, 4th form Frieza from Super- post training -should be able to literally manhandle him with the smallest bit of effort.
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