Well, Batman is just a guy with a lot of money so is Iron man and spiderman is just a kid who got bite by a radioactive spider, and the hulk has radiation poisoning. Every superhero when you dig deep enough are just ordinary people who had something extraordinary happen to them.
Steve and Bucky
Steve and Natasha
Bucky and Natasha
Loki and Natasha
Natasha and Bruce
Clint and Natasha
I'd say Natasha and Bruce