There are many great and seminal albums.
I can second the Allman Brothers "Fillmore East/Eat a Peach" and Kinks "Village Green" listed above.
For sheer live power, The Who's "Live at Leeds" captures the band in top form. This album is often over shadowed by "Tommy" and "Who's Next" in "best of" shout outs by the press, but remains one of the best live artifacts of the 1960s. This "pre-metal" sound was in direct contrast to the Summer of Love vibe that informed most song structures of 1968 to 1970 (i.e. Grateful Dead, Janis Joplin). The deluxe edition of the CD includes the entire "Tommy" set, the first rock opera, plus a nice mixture of rare early songs ("Fortune Teller," "Tattoo")used to warm up to. Keith Moon's drumming is superb on this and gives the songs true meaty weight. Led Zep duly listened to this and crafted a similar sound in the 1970s that becam heavy metal.
The Small Faces, and the later Faces, pioneered a sound not too dissimilar to today's alt rock. This was sloppy bar band rock at its finest. The Faces had four consecutive albums (1969 to 1973) of brilliant journeyman playing and Rod Stewart never sounded better as a lead singer. Rhino has a definitive 4 CD set called "Five Men Walk Into a Bar." They were often call a pub man's Rolling Stones. Speaking of Jagger and Co., The Rolling Stones had an incredible "later" decade from 1966 to 1970. "Let It Bleed", from 1968, captures the band at their height--these songs still remain the core of their live set. American music anchored in country and blues never sounded so good done by English musicians, Faces or Stones.
The Band produced two brilliant albums, "Music from Big Pink", and "The Band". No one did Americana better, and these still sound incredibly fresh today.
The Beatles work are well documented. I think the first solo albums are under-rated, from 1970. George Harrison's first album, "All Things Must Pass", and Sir Paul's first two albums, "McCartney" and "Ram", are interesting albums that are simple in their execution but feel miles of away from the weight of being legends in their first band. They sound like they found the joy of making music again.
Iggy and the Stooges pioneered what became "punk rock", a full six years ahead of the curve. Match this with the Velvet Underground, and you can experience music that was brutally honest and inspired so many countless later bands, from The Clash to REM.
Jimi Hendrix set the standard for guitar playing and all of his albums are interesting to reexamine. I personally like the experimentation of the third one.
