If you're determined to only watch one Cisco Kid movie, or if it's important for you to start with the best and work your way down, watch Romance of the Rio Grande with Cesar Romero as the famed and lovable bandit. It's the best one by far, and the only one of the series I've seen that doesn't fit in the mold of "one of many" like the others do.
When Cisco and his sidekick Gordito chance upon a stagecoach, they're not the first bandits on the scene. Already bad buys have killed the driver and wounded the passenger riding shotgun. When they turn over the wounded man, they're shocked. It's Cesar Romero! He gets to play a double part in this movie, which is very exciting. Two heads of gorgeous wavy hair, two blinding smiles, two slim figures in tight pants - yes, please! It turns out, the dying man is a visiting grandson to a wealthy land owner in a nearby town. Cisco has the idea to impersonate his lookalike, get some money out of the old man, and leave town with Gordito.
But, when he arrives in town, everything changes. He's treated with love and kindness by his "grandfather", he meets a sweet girl Lynne Roberts, and he has a change of heart. He also has a change of accent, because his lookalike was visiting from Portugal. It's so much fun to see him in a double part, not to mention the trick photography to put both the Cesars in the same frame! I know Warner Baxter won an Academy Award for playing the Cisco Kid, but Cesar's my favorite. Let alone two of him!