Surrealistic Pillow, for God's sake, Surrealistic Pillow. It's never the favorite of hardcore Airplane fans (Myself included), but it's by FAR the most accessible to someone who's not really "into" the airplane - it's the perfect Gateway drug. Volunteers is also a really good pick, but I think you should get Pillow first - I honestly found Crown of Creation more inviting than it, at first, though. I couldn't really "get" Baxter's or Volunteers until I'd been listening to Pillow and Crown awhile.
Also, if you can find the relatively rare "Live at The Monterey Festival", it's an excellent intro to the Airplane - you get to hear a lovely "Today" which Grace playing organ, some really good folk covers with Marty still sounding fresh and enthusiastic, a really energetic version of "She Has Funny Cars" and probably the best "Ballad of You & Me & Pooneil" ever recorded. So for the God of Grace, BUY it if you come across it.