Out of the known "flame" ("heat") DFs, the only confirmed superiority is magu magu > mera mera (SBS Vol. 79). However, I'd guess that magu magu is the superior overall since the Vivre Card Databook (Akainu's card, #0595) has the info that magu magu possess the highest offensive power of all DFs.
Speculating the rest, I'd put like this: magu magu > uo uo (seiryu) > tori tori (phoenix) > mera mera > netsu netsu.
Edit: oh, and putting King's and Sanji's flames, I guess it goes like this: magu magu > uo uo (seiryu) > ifrit jambe ~ tori tori (phoenix) > mera mera > King's flames ~ diable jambe > netsu netsu.