According to the article in murraymints' link, ZZ Top have not only the longest lasting line-up, but the ONLY line-up that consists of the original members.
Actually, that's not true about Rush as John Rutsey was the original drummer on the first album and not the incomparable Neil Peart who's been on the stool since.
Nope, Status Quo have been constantly changing their line up since John Coghlan left in the early '80's (and even before that there was a keyboard player (Roy Lynnes??) who left just before the band hit big in the early '70's)