This Is Spinal Tap is very iconic spoof of a documentary of a rock band, but what band or bands is Spinal Tap actually based on?
There is no actual answer to this question. Spinal Tap, believe it or not, began in the mind of Rob Reiner (Meathead, if you're an All In The Family fan...) in 1979. There was a reference to the band in a skit he wrote for a show called "The T.V. Show" which aired on ABC.
It is said that some aspects relate to certain real-life events (i.e. the drummer choking on vomit was probably a reference to John Bonham from Led Zeppelin), but Rob Reiner to the best of my knowledge has never confirmed nor denied any of it.
'Tap are just based on astute observations of rock bands and the lifestyle in general, not any one band in particular. There are plenty of references through out the film, e.g. death by vomit ala Jimi hendrix, Bon Scott, John Bonham; the band getting lost in the stadium at Cleveland is a reference to a real life documentary which includes a scene of Bob Dylan getting lost trying to leave a concert venue and failing to find the exit.
Many also draw connections between "This is Spinal Tap" and "The Bad News Tour" from The Comic Strip Presents (1983). But as stated previously, it likely drew influence from a number of musicians and comedians.