Houses of the Holy is Led Zeppelin's finest hour.
The previous untitled album is better known and one of the all-time best sellers in music history, but it refines the Zeppelin attack and contains everything great about the group. Far from simple hard rock, this may also be the most diverse Zep album, stylistically, and is the best example of the humor, power, emotion and raw thunder that Zeppelin still is known for. 
"The Rain Song," for example, is a powerful piece that starts off with a gentle acoustic strum, adds on Robert Plant's vocals for a while and then segues into some gorgeous guitar work. Halfway through, the drum and bass kicks in, but it's never overpowering, rather adding to the whole feel of a cloudy-day epic. Few bands could write songs this intricate and pull them off. "No Quarter" is powerful as well, letting John Paul Jones shine with his keyboard work and inspired by his time on the road away from home; the ominous sprawl and Plant's howling voice in the chorus transports you to a different place.
"The Song Remains the Same" is among the best instrumentals ever written by the band; unfortunately, it didn't end up that way because of Plant's vocals, which kill the momentum built up each time by the music. Still, as a road-trip travelogue, it's better than most. "Dancing Days" is better, an ear-bleeding shred that envelops you in its haze, yet never forgets to carry the melody. "Over the Hills and Far Away" is the only acoustic song here, but it's one of Zeppelin's best, taking its time to unfold its many layers before breaking into an upbeat lite-rock song halfway through.
The reggae-inflected "D'yer Mak'er" and the funky "The Crunge" sound like little else in the Zeppelin catalog; they may not be classics, but they are fun, and prove that all of Zeppelin's imitators could never quite get the diverse musical mix or subtlety. Of course, the closing "The Ocean" has a bar-band charm and powerful Bonham drumming, not so much for the noise but for how he is able to pause for each bar during the verses, playing the drums like a lead instrument.
Zeppelin never topped this album; it remains their peak and a cornerstone of classic rock. It's the moment where all of Zeppelin's vision and musical integrity came together in a grand way.