Hard Rock: when mentioned, it immediately brings to mind Led Zeppelin and Deep Purple (as well as Cream and Free)... I would like to propose a few examples of less known groups or those not directly associated with the genre.
20 - Ohio (Live)
C S N Y four artists, both together and as soloists, very versatile: capable of "dressing" each song according to the circumstances (live, in studio) and the instrumentation (acoustic, electric) in different ways. This is one of their songs in the most "Hard" version...