On the orchestral side, check out The Yellow Shark, performed by Ensemble Modern. Frank himself, sadly as one of his last projects, spent a lot of time working with this group (though not playing), I believe it's all live (various venues), and in my experience everyone from young children to grandparents really like it.
I think the London Symphony stuff was a bit weak, but haven't heard that in ages. I've got a recording by the Omnibus Wind Ensemble, 'Music By Frank Zappa', which is a solid performance and beautifully recorded, but I don't pull it out very often... If you want FZ's compositions played by an orchestra, go for the shark.
And if you like the compositions, and maybe the perspective on life, but don't necessarily need the guitar playing, Civilization Phaze 3 is pretty intriguing.
On the Dynamo Hum side of things, how can you ignore the Mothers Live at the Fillmore in 71? I only listen to it every six months or so, but this one has a permanent slot in the CD changer in my car - sometimes I just need it.
We could go on (and probably will)...