There are very few things in this world that are perfect; and even then those things rarely last long.
There are "greater" rock albums, and "greater" bands than Deep Purple, and their third album "Machinehead"... but it is an absolutely perfect album from an absolutely perfect band.
The Mark II lineup of Deep Purple; Ian Gillan (vocals), Ritchie Blackmore (guitar), Jon Lord (keyboards), Roger Glover (bass guitar) and Ian Paice (drums); was literally perfect. There could not have been a better band possible at that moment.
Sure, Clapton and Page are a little bit better than Blackmore; Mercury and Plant a little bit better singers than Gillian; Bonham and Moon a little bit better drummers than Paice; but could you imagine a band with better talent in EVERY position like this?
Then they produce what is the ultimate hard rock album of the 60s and early 70s. There isnt a second of that album that isn't truly perfect, and it was released at exactly the perfect moment.
So what other perfect albums are out there, from the perfect band (or artist), at the perfect time?