JUST WHO is the single biggest character in the Marvel Cinematic Universe? Ask a typical comics-convention line and the most likely answer is Robert Downey Jr.’s Iron Man. Tony Stark lights up every Marvel scene he’s in, whether he’s armored up or not. There’s a reason why Marvel pays mightily to ensure each return of Downey’s breakthrough superhero.

But what if we reframe the question to ask: Which Marvel Studios character has the best solo movie franchise?

Well, with Friday’s domestic release of “Captain America: Civil War,” the film’s title character will surely move to the top of Marvel’s cinematic totem pole. The Star Spangled Avenger’s new movie might even match the mega-events that are “Avengers” movies.

Of course, “Civil War” essentially is a team-up movie, too, but there’s a reason the filmmakers chose Cap to build such an event around.

At the center of the Cap-franchise success is actor Chris Evans. He has grown into being a perfect fit for the role, despite his initial uncertainly over accepting the role. (Evans had already played a Marvel hero on screen, of course, as the Human Torch in the dated “Fantastic Four” movies.) Marvel’s persistence in landing Evans to play Steve Rogers over multiple pictures has certainly paid off. The studio knew Evans was its Captain; it just needed Evans to figure that out.

(Rewatch Evans’s “Fantastic Four” performances now, by the way, and you can appreciate Evans’s underrated range. As Johnny Storm, Evans was a free-flying, hotheaded pretty boy who took nothing seriously — the polar opposite of the straitlaced moral leader that is Captain America.)

Marvel and Evans, of course, have had to grow and mature the character’s franchise. 2011’s “Captain America: The First Avenger” mostly served its purpose, telling its World War II origin story (and introducing key MCU characters Agent Peggy Carter and Cap’s best friend, “Bucky” Barnes).

Adding to the glow of the Cap franchise, though, is the consensus that its second film, 2014’s “Captain America: The Winter Soldier,” is better than its first film, and early reviews suggest that the third installment might well be even better yet. Contrast that with the “Iron Man” franchise, in which the first film, it’s generally agreed, is the best in the Tony Stark trilogy.

The Captain America franchise has been buoyed, too, by the coming-aboard of the Russo brothers to steer the proceedings. Under Joe and Anthony Russo — who direct “Civil War,” too — the second film took on darker and more political tones than most MCU outings.

“The Winter Soldier,” as I’ve stated before, might well be the best Marvel movie ever. Yet if “Civil War” can surpass “Winter Soldier” creatively, it would achieve the uncommon feat of a third film in a franchise being the best of the bunch.

Marvel smartly continues to foster its next generation of cinematic heroes. But for now, Captain America remains the biggest gun as the best solo Avenger on film, given his cinematically chiseled body of work.

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