I like Capra. Now, I'm not saying I would include him in my list of Top 10 filmmakers. But, he'd probably make my Top 25.
Anyway, the answer is It Happened One Night, for my money Capra's best. Made in 1934, it starred Clark Gable and Claudette Colbert. It also won best screenplay, making it a Big 5 winner.
It's one of my personal favorites. The first "screwball" comedy, it was the inspiration for such films as My Man Godfrey, Bringing Up Baby and The Lady Eve, all great movies.