Heist movies are one of my favorite genres. At their core, they're about as formulaic as you can get: assemble the gang, plan the job, pull off the heist, getaway or die trying. But the simple structure lends itself to endless variations on the theme.
The late 50s have at least two classic heist movies. "The Killing" is famous in large part because it's early Kubrick and scripted by Jim Thompson. I haven't watched it recently and maybe I'm dead wrong but I'd rate it as good but not great. On the other hand, "Rififi" is a contender for the GOAT of the genre. "Rififi" excels in all segments of the classical heist plot structure. The pre-caper phase assembles an interesting set of characters. The robbery sequence is a masterful 20-30 minutes of suspense without dialog or music. This occurs relatively early in the film which gives plenty of time for things to unravel for the gang, which may be the best part of the film.
My pick for an underrated 1955-59 heist film is "Odds Against Tomorrow", a 1959 movie starring Harry Belafonte, Robert Ryan and Ed Begley. The film's central gimmick is that the robbery forces Belafonte to work with an loathsome racist character played by Ryan. Belafonte was one of the few African American actors of the era who could carry a commercial Hollywood movie but it's still a pretty low budget affair. That works in its favor though with a gritty black and white location footage helping to define the desperate characters. Where "Rififi" places the robbery sequence near the middle of the film, "Odds Against Tomorrow" spends most of its running time on character development. The heist and post-heist action happens right at the end. The robbery itself is nothing special but there's still a lot to like. Ryan is terrific, Shelley Winters and Gloria Grahame get a few scenes each to help establish Ryan's character, there's a nice jazz score by John Lewis of the MJQ and director Robert Wise does a good job of establishing tone and space. The script was co-written under a front by blacklisted writer/director Abraham Polonsky. It's telling to contrast the Communist Polonsky's acerbic take on the state of late 50s race relations with the more uplifting message for White liberal Stanley Kramer in "The Defiant Ones".
There's a decent three part print of "Odds Against Tomorrow" on YouTube. It's certainly not in the same class as "Rififi" and probably not as good as "The Killing" either but it's an interesting hybrid of heist movie, late-era film noir and social commentary.