All great in their own regard.
M. Jackson - An innovator in vocal styles, dance and videos. Many have come since but none have done what he did as well as he did it.
Beatles: Songwriters! Also, the first to gain massive public exposure with/for the distorted guitar sound. There have been better musicians and better bands and better sounds, but their songwriting and use of vocal harmonies stands even today. Perhaps the greatest argument for the Beatles legacy is the sheer legions of future musicians, successful and otherwise, that note the Beatles and their music as a primary influence.
Elvis Presley: One of the first "eye candy" starts. Had his own sound and decent songs and pushed the envelope of pop music early in his career, but ultimately he capitalized on looks that brought women to their knees to enable a major career in music and movies. Definitely contributed to music, but without the looks, it's argueable that his career wouldn't have been as big or as long. That said, he did open the minds of the world a touch and that made the public much more open to the music that followed him.
Rolling Stones - Gained a huge following early on when they were pushing the boundaries of the scene. Richards was arguably on the cutting edge early on but became unimpressive as music and musicianship eleveated around him. Jagger, brought little vocally, but made up for it, apparently, with his live performance in energy and style along with keeping himself and the band in the spotlight through his purported sexuality ambiguity and sexual indescretions - hardly anything that should justify the term "greates". Again, this is a band that's best attribute was songwriting, although I've never been a particular fan of anything Rolling Stones.