Alright. I’m going to have to vote for Sanji then.
Sanji has comparable, if not superior heat resistance than Brook, as shown when comparing their reactions to the explosions from Pacifista lasers (
here). Brook was able to survive being coated Nami’s lightning without any issues (
here). Lightning’s is 27760°C, meaning Sanji can withstand the heat of Bakugo’s explosions without issue.
As for actual combat, as far is I’m aware, Bakugo rarely uses his abilities to outrange his opponents from attacking from afar or air camping, but he more so rushes into his opponents with his explosions to attack them with it. The issue with this fighting style is that Sanji hard counters.
Sanji is extremely smart, being called the chief strategist and brains of the crew in several occasions (
here) which would put his tactical ability and fighting smarts at least on the level as people like Luffy, Zoro, Nami, and Usopp, if not greater. You can read Zoro and Luffy’s intelligence on their own pages, but Nami was stated to one of the smartest characters in the series, and her adaptability is off the charts, having one of the best deductive reasoning skills in the verse and being able to make split decisions in mere moments.
He’s been stated to not rush into fights and come up with several battle plans in order to win with each of them leading to his win (
here), meaning that even if Bakugo somehow manages to surprise Sanji, he’d still have several other ways to take him down, which is supported with this statement (
here).
Alongside his intelligence, Sanji has equally impressive fighting skills, with his entire moveset being centered around accuracy, power, agility, speed, and breaking down his opponents. Here are just some of his kicks and what they do (
1,
2).
Given how Sanji is both smarter, more skilled, and resists the heat from Bakugo’s explosions, the only thing that Bakugo can rely on is his superior durability, which is already negligible, but that gets negged by Sanji’s Flanchet Shoot, which has durability negation.
For these reasons, Sanji wins mid difficulty at most.