Captain America: The First Avenger

Everything else in this run happens after this. Before the labs, the funding fights, and the men in expensive suits arguing about who gets to own the future, there's a scrawny kid from Brooklyn who keeps getting rejected for service and keeps trying anyway — not because he thinks he'll win a fight, but because he can't stand to watch one and do nothing.

The formula that follows is almost beside the point. The film spends far more of its runtime on who Steve Rogers already was before anyone changed his body than on what he becomes after. That's the right call, and it's why watching this first, ahead of everything it precedes, changes how the rest of the saga reads: every later film's "legacy" and "greater good" arguments are being measured, quietly, against a guy who volunteered for none of it and would have done the same thing at half his eventual size.

It's also, structurally, a war film first and a superhero film second — grimy, practical, more concerned with a platoon than a universe. Worth remembering once the scale gets bigger later: this is where it started, and it started small.