These legends have made invaluable contributions to this immense world of music and provided a new direction of it. In their era, huge mass followed them and their music and that haven't changed in present as well. Whatever they wore became a fashion and whatever they sang became the voice of everyone.
The greatest four "J"s in the history of music according to me (arranged alphabetically) are:
1. Janis Joplin: Janis Lyn Joplin (January 19, 1943 – October 4, 1970) was an American singer, songwriter and music arranger. She rose to prominence in the late 1960s as the lead singer of Big Brother and the Holding Company and later as a solo artist. In 2004, Rolling Stone magazine ranked Joplin number 46 on its list of the 100 Greatest Artists of All Time, and number 28 on its 2008 list of 100 Greatest Singers of All Time.
2. Jim Morrison: James Douglas "Jim" Morrison (December 8, 1943 – July 3, 1971) was an American singer, songwriter, poet, writer and filmmaker. He was best known as the lead singer and lyricist of The Doors and is widely considered to be one of the most charismatic frontmen in rock music history. He was also the author of several books of poetry and the director of a documentary and short film. Although Morrison was known for his baritone vocals, many fans, scholars, and journalists have discussed his theatrical stage persona, his self-destructiveness, and his work as a poet. He was ranked number 47 on Rolling Stone's "100 Greatest Singers of All Time"
3. Jimi Hendrix: James Marshall "Jimi" Hendrix (born Johnny Allen Hendrix; November 27, 1942 – September 18, 1970) was an American guitarist, singer and songwriter. He is often considered to be the greatest electric guitarist in the history of rock music by other musicians and commentators in the industry, and one of the most important and influential musicians of his era across a range of genres.
4. John Lennon: John Winston Ono Lennon (9 October 1940 – 8 December 1980) was an English rock musician, singer-songwriter, author, and peace activist who gained worldwide fame as one of the founding members of The Beatles. With Paul McCartney, Lennon formed one of the most influential and successful songwriting partnershipsof the 20th century and "wrote some of the most popular music in rock and roll history". He is ranked by Billboard as the second most successful songwriter in singles chart history after McCartney.
Although I needed to mention only 4 names, I couldn't afford to miss out "Jimmy Page" from this list. James Patrick Page (born 9 January 1944) is an English guitarist, songwriter and record producer. He began his career as a studio session guitarist in London and was subsequently a member of The Yardbirds from 1966 to 1968, after which he founded the English rock band Led Zeppelin.
Short Biographies taken from wikipedia