10. James Rhodes
As Tony Stark's best friend and right hand man, James Rhodes has been a key part of the Iron Man franchise for decades. After Tony hung up his iron-plated underoos in the 80's following a relapse into alcoholism, Rhodey was the natural choice to replace him.
However, he never really seemed to inherit the role and was relegated to the West Coast Avengers team with D-Listers like US Agent and glorified janitor Stingray. It was clear that Rhodey wasn't a true successor, just a stand-in until Tony got his act together.
He eventually got his own suit of armour though and carved out a niche as a new superhero. With a less flashy black and grey colour scheme and packing more heat than the average Trump voter, War Machine proved to be very popular with fans, to the point that many were crying out for him to make his big screen debut almost as soon as the first Iron Man film was announced.
War Machine has had numerous ongoing series over the years and, while never reaching the same popularity as Iron Man, he's proved to be one of the most popular and enduring characters in the Marvel Universe... even though he's currently taking some time off to be dead.