10. War Machine
Real talk, Terrence Howard is actually really good as James Rupert "Rhodey" Rhodes in the first Iron Man movie. Given this was the MCU's first official outing and there was nothing but the weird Hulk films and Fantastic Four messes to judge it by (Maguire-era Spider-Man doesn't count because Maguire-era Spider-Man was a law unto itself) he put down one of the strongest Marvel supporting roles we'd ever seen.
Even his very cheesy War Machine tease of "next time, baby" in the film's final act got the response the studio wanted. It hinted at a wider universe of material to pull from, and let fans know that there would be plenty more of these movies in the pipeline. However Howard arrived as the movie's highest-paid actor and, following the massive pay rise Robert Downey Jr was due for the sequel, was pushed out in favour of Don Cheadle.
Marvel would have been taking a huge gamble in predicting the next 10 years of the MCU's critical and financial success but, in hindsight, they'll wish they'd cast for the long-term rather than the short. Given how Cheadle has defined the role in the subsequent movies, they'll have wished dearly they could have introduced it with him.