I will answer in a totally biased unbiased fashion if this is at all possible because this highly emotionally charged debate is divided along generational lines.
The teenagers of the fifties will say Elvis and this teenager of the eighties must say Michael Jackson. After all, Michael Jackson is my Elvis. But as in the boxing world one usually asks the question, "who's the best boxer pound for pound"? I will coin my own phrase and ask a question of my own, "Who was the better artist talent for talent"? In my opinion there only can be one answer, and the question is now a rhetorical one because it must be "His Royal Highness- The King Of Pop" .. Michael Jackson was a child prodigy blessed with more than average talent by the creator and was already a veteran of the music business by age twelve and to preserve objectivity we will first look at the similarities between the two kings and then explain what makes Micheal bigger.
Elvis broke big at the advent of a new technology called television. Micheal gained super-stardom at the start of music television programming called MTV. Vocally Elvis truly outclassed other rock and rollers. Vocally Michael Jackson outclassed other black singers. Elvis as a live performer changed the face of rock and roll. Micheal as a live performer changed the face of popular music. But seriously though I am being really kind, too kind to Elvis. In his eulogy for MJ , Berry Gordy, founder of Motown Records said this, "Michael as an artist and entertainer not only raised the bar he broke the bar" This is so true as MJ shattered every record and dismantled every stereo type of the black performer and became the standard that all others reach after. Who has the biggest single selling album of all time? It's not Elvis. It's Michael with the musical benchmark called "Thriller". Elvis built his career by covering other peoples material. Michael did not do covers because he never followed, he led. He [MJ] did not piggyback trends he was the trend
Michael's real legacy is that he was a true original in the purest form . He acknowledged, embraced and enforced his uniqueness to the point of weirdness. He was not ordinary and he made damn sure the world knew it. His vocal range was at least two octave and he was the master of vocal originality ,creativity and ad libbing. Visually he became the template for live performing creating his own dance moves and then some and along the way became a fashion icon with his attire on and off stage. But when all is said and done, Michael left us the music. The sheer volume and the quality of which Elvis didn't even come close to and which will be hard to duplicate even by artist who are still living. Elvis made good music but his career went into decline relegating him to a Vegas act. Michael, with that one album, "Thriller", transformed and rescued the then ailing music business. In songs like "Beat It" he made the hard rock song every rocker could be proud of. In "Billie Jean" the pop gem every artist dreams of writing." Wanna Be Starting Something" must be one of the best dance songs ever and "Lady In My Life" is a love song that is up there with the other classics. We should also remember that he revived the career of the aging Beatle Paul McCartney with their duet, "The Girl Is Mine" and also blew the shock rockers out of the water with the title track. Sorry Elvis the pelvis, that was a period when a musical genius touched the face of brilliance that even you would have had to admit was bloody awesome.
As for cultural impact there is a whole plethora of artist such as Backsreet Boys, Justin Timberlake, Usher, Neo and many others who are or have used the Michael Jackson mold to become successful. The worldwide reach and influence of MJ is a phenomenon in itself and music critics and journalist far and wide have already admitted that Michael's posthumous earnings will dwarf that of Elvis's. In every culture and nation on this planet. I strongly believe that more people want to be like "Mike", instead of Elvis.