For me, Hot Rats, Uncle Meat, Burnt Weeny Sandwich, Weasels, Absolutely Free (for nostalgia's sake; I was 12 when I first bought it), and Fillmore East (absolutely brilliant satire). If you want to narrow it down to one or two, Hot Rats and Uncle Meat.
Thanks. What about the other live one -- band from LA or somesuch?
Just Another Band From L.A....Billy The Mountain (Side One) is EPIC. I'm sure Side Two was good, but frankly, I can no longer remember it.
My reaction's the opposite: BTM tries to make some sort of point, but comes off as scattershot satire. But side 2 has a lot of energy and plays really well...a killer Call Any Vegetable into Eddie Are You Kidding (wonderful Flo & Eddie on this one) into Magdalena (lyrics not for the squeamish, however) ending with some powerful Dog Breath.
I didn't recommend JABFLA because many people consider the Flo & Eddie band to be low-level satire and sex jokes. Of the two albums in this batch (3 if you include Chunga's Revenge), I think Fillmore's the strongest and most consistent. My guess is that JABFLA might not have been released if it wasn't that Zappa needed money to pay medical bills following his fall.
I'll be curious about Chunga, because the original LP sounded very muffled; I wonder if they'll be able to improve it.