(100% Fed) Galactus vs. (Full Phoenix Force) White Phoenix of the Crown vs. (Pre-Retcon) Molecule Man
In terms of environment they all have neither an advantage or a disadvantage, the battle takes place in a neutral empty area in space. No prep for anybody, they all just happened to randomly stumble upon each other when they were each at the strongest they ever were. This is a fight of basically pure ability/power/wit/skill.
Bonus question: What would happen to guys like the Avengers (Batman, Thor, Hulk, Cap) or guys like Wolverine and Spiderman (you know your ordinary everyday superheros that people recognize) if they were to get caught in this fight or to try to participate in it. Would any of them have a fighting chance or would they get absolutely obliterated?
I don't know a whole lot about what these guys could do if they were at full power so please don't tell me I'm trying to spite someone if it turns out that one of these guys is clearly better at full power. I'm genuinely curious.