Answer by taking a quote from the following article:

ZZ Top  is an American rock band formed in 1969 in Houston, Texas. The band has, since 1970, consisted of bassist and lead vocalist Dusty Hill, guitarist and lead vocalist Billy Gibbons (the band's leader, main lyricist and musical arranger), and drummer Frank Beard. "As genuine roots musicians, they have few peers", according to former musician, critic and collector Michael "Cub" Koda. "Gibbons is one of America's finest blues guitarists working in the arena rock idiom [...] while Hill and Beard provide the ultimate rhythm section support."

The original line-up was formed in Houston and consisted of Gibbons, organist Lanier Greig (died February 2013) and drummer Dan Mitchell.  The name of the band was Gibbons' idea. The band had a little apartment covered with concert posters and he noticed that many performers' names utilized initials. Gibbons particularly noticed B.B. King and Z.Z. Hill and thought of combining the two into "ZZ King", but considered it too similar to the original name. He then figured that "king is going at the top" which brought him to "ZZ Top".  ZZ Top was managed by Bill Ham, a Waxahachie, Texas native who had befriended Gibbons a year earlier. They released their first single, "Salt Lick", in 1969, and the B-side contained the song "Miller's Farm". Both songs were credited to Gibbons. Immediately after the recording of "Salt Lick", Greig was replaced by bassist Billy Ethridge, a band-mate of Stevie Ray Vaughan, and Mitchell was replaced by Frank Beard of the American Blues. Due to lack of interest from U.S. record companies, ZZ Top accepted a record deal from London Records. Unwilling to sign a recording contract, Ethridge quit the band and Dusty Hill was selected as his replacement. After Hill moved from Dallas to Houston, ZZ Top signed with London in 1970. They performed their first concert together at the Knights of Columbus Hall in Beaumont on February 10.  In addition to assuming the role as the band's leader, Gibbons became the main lyricist and musical arranger. With the assistance of Ham and engineer Robin Hood Brians, ZZ Top's First Album (1971) was released and exhibited the band's humour, with "barrelhouse" rhythms, distorted guitars, double entendres, and innuendo. The music and songs reflected ZZ Top's blues influences. Following their debut album, the band released Rio Grande Mud (1972), which failed commercially and the promotional tour consisted of mostly empty auditoriums.

what was their greatest hit?