I remember asking my mate, a Zappa completist, the same question a few years ago. He actually rang me back and asked me if I had any idea what I was getting myself into! Haha!.
Anyway, he suggested The best of Frank Zappa, as a taster, which I did. Then I picked up Hot Rats, Apostrophe and Over-nite Sensation. His repertoire is mind-blowingly broad, but great fun. I personally like The Grand Wazoo a lot.
Once you’ve really listened to a bit of Zappa you realise what a musical bloody genius he was and he’ll always be one of your favourite artists. He’ll also keep you busy discovering music for years!
Enjoy! As Terrahawk said, you can’t go wrong with Zappa.