With such a large catalog and so many hits, it's difficult to declare one song Elvis Presley's calling card. Still, you could make the case that "Heartbreak Hotel" is that song.
"Heartbreak Hotel" was the first song Presley recorded with RCA records and it was his first number one hit. The song was the first in a series of singles (including "Hound Dog") released in 1956 that would turn Presley into music's biggest star. It was the song that introduced him to a national and international audience, and the song that introduced him to aspiring musicians, like Keith Richards, who would cite Presley as their greatest influence.
"Heartbreak Hotel" is also one the few songs on which he shares a writing credit—although, as with the others, he actually played no part in the song's writing.
"Heartbreak Hotel" is a good example of Presley's ability to transform the conventional genres that influenced him. The song is built on a basic eight-bar blues progression, so in the hands of a more conventional artist, it could have been straight blues number. Instead, Presley gave it an edge and an attitude. He converted a potentially plodding and morbid lament about heartache into a snarling, pulsing piece of rock and roll.
In other words, "Heartbreak Hotel" summarizes Presley's real contribution to music, which was his ability to fuse several genres into a distinctive new sound. Presley drew from the blues, country, and gospel music to advance the development of rock and roll. He wasn't alone in this experiment, but he mastered the formula and delivered it successfully to huge audiences.