Rick,You've opened a real can of worms here. Whenever people ask for opinions about the "best" Zappa albums they get a wide divergence of suggestions because he produced so many different kinds of music -- the early silly yet brutal "comedy" Zappa, the sophisticated jazz/rock Zappa, the Flo and Eddie Zappa, the guitar Zappa, etc.
By way of background (and giving away my age), I stumbled onto We're Only In It For The Money in about 1969, when I was in 9th Grade. I was stunned. I bought (and still have and play!) every album that came out thereafter (and before!) and attended at least one concert every time he came through NYC -- whether at the Fillmore East, the Palladium or the Felt Forum -- until his death.
When it comes to suggesting an album or albums, it's very difficult because my associations date back to 1969 or 1975 or whenever -- associations that will mean nothing to you. That said, I'll suggest a few, pointing out my biases. Take it with a pound -- not just a grain -- of salt.
Early Zappa/Mothers -- We're Only In It For the Money (1968). A hilarious yet brutal take on American Culture. If you like this, try Freak Out and Absolutely Free.
Early "Jazz/Rock" Zappa -- Hot Rats (1969). If you like this, try Waka/Jawaka and The Grand Wazoo.
Early Comedy/Guitar/Flo and Eddie Zappa -- Live At The Fillmore East June 1971. A personal favorite because I attended several of these. Incredibly nasty skits (warning: some indecent but hilarious material) and amazing and amazingly sophisticated music. If you like Flo and Eddie, try Just Another Band from LA (1972).
Middle Zappa: Apostrophe (1974), which you cited. Just a great all-round album. FWIW, probably my most-played album during college. If you liked this, I'd suggest Over-Nite Sensation (1973), One Size Fits All (1975), Roxy and Elsewhere (1974) and (a little more distantly) Zappa in New York 1978.
A Bit Later: Joe's Garage Parts I and II (2 albums, 1979), which you also mentioned -- Both biting social commentary and heavenly guitar. "Watermelon in Easter Hay" (from Part II) contains one of the great guitar solos ever played. If you like the "guitar" Zappa (and live his guitar solos were simply overwhelming) there are several guitar solo collections such as "Shut Up 'N' Play Yer Guitar".
Happy listening!
e