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Epic Riff if it's a song can be identified immediately just from that snippet
Signature Line if it's a particular lyric (Please allow me to introduce myself/I'm a man of wealth and taste...)
Big Rock Ending for something like the end of Freebird where it just pulls out all the stops
Something Something Leonard Bernstein if it's the only part of the song that people know, Chorus-Only Song if most people don't even know there's more to the song.
For completeness, I'll add Siamese Twin Songs for paired songs like "Sirius" and "Eye In the Sky" from the The Alan Parsons Project- a song in two halves, and both are widely known and recognizable. You'll rarely hear them played together, however, as one is a fairly epic example of instrumental prog rock, and the other could best be described as a soft rock anthem.

This is like Signature Scene, but for a song. It's widely agreed to be the best part of the song, even if the track is great to begin with.