...you are missing the point!
As a self-confessed Zappa bore who owns every CD he ever released (and maybe a few more he didn't!) I find it hard to recommend just one album. "We're Only It For The Money", however, should be in any 60's rock fan's collection as representative of the era while mocking the culture that grew up around in it. Zappa once claimed he was merely an anthropologist and on this album picks apart many of it's subjects.
The songs make fun of the hippy dream and in particular anyone who appeared to be hopping on the bandwagon (Flower Punk, Absolutely Free). Zappa also picks on the parents of flower children (Bow Tie Daddy, Mom & Dad) and the freaks and perverts who hung around them.
Despite his obvious cynicism Zappa wrote some of his best, and yes, even prettiest tunes for this album. A few numbers even reflect on the more serious social implications of the era. The final number, "The Chrome Plated Megaphone of Destiny" is a sound collage that pre-dates Zappa's more serious work he produced later in life.
So if you must only have one Zappa album "We're Only In It For The Money" is a good choice, or a good place to start...