Instead of fighting, every member of the Avengers and Guardians of the Galaxy(+ Loki, Ultron, and Thanos, because why not?) will instead have to cook a three course meal for a panel of impartial judges who will appraise them on the quality of the food and the creativity with which it is presented. They have three days to produce whatever they want with no outside help(And they cannot use their powers for coercive purposes). Who impresses the judges with their culinary abilities?
I have a feeling his diet consisted mostly of pizzas, cheeseburgers and hotdog. Don't ask me why, just a gut feeling.
Anyway to answer the question, we can at least eliminate a few contenders:
Vision is out - we know he sucks at cooking as shown in Civil War.
Steve Rogers is probably out - he's too used to soldier rations plus he boils everything.
Tony Stark is probably out - rich kid like that probably had someone else always cooking meals for him. Same goes for Loki and T'Challa.
The badass bachelor/bachelorette-types are probably not too good at cooking either considering their lifestyle. So that takes out Starlord, Bucky, Gamora, Nebula, Rocket, etc.
So what does that leave us?
Well, it might be one of the family men. Either Antman, Hawkeye or Drax.
It might be one of those used to caring for someone else, like Peter Parker or Mantis.
Personally I think it's Black Widow. Simply because I think culinary arts were probably part of the skillset she was taught to become a convincing undercover spy.
I was thinking about this. This was most likely true pre-Endgame. But Tony has had 5 years of domestic family life, living seemingly out of the way with Pepper and his daughter. Hell, IIRC, he was doing the dishes just before he solved safe time travel lol. So, it's quite possible that he learned to cook in those 5 years as well. Small children can be very stubborn and demanding.
__________________ Never take life seriously. Nobody gets out alive anyway.