Although I think that Zappa's best period was in the early to mid 1970's, his groundbreaking earlier works with the Mothers of Invention should not be forgotten:

*Freak Out
*Absolutely Free
*Lumpy Gravy
*We're Only In It For The Money
*Cruising with Ruben & The Jets (Great album in 1968, but the 1980's remastering on the CD reissue sucks)
*Uncle Meat
*Burnt Weeny Sandwich
*Weasels Ripped My Flesh
*Fillmore East June 71
*Just Another Band from L.A.