I'm going to vote for Meat is Murder...it has tracks like "The Headmaster Ritual", "How Soon Is Now?", and "That Joke Isn't Funny Anymore", and I think it's their most challenging CD. I personally sort of dislike their eponymous debut, though it has "This Charming Man" and "Hand in Glove" on it, it also has alot of dull, same-sound tracks like "Pretty Girls Make Graves." The Queen is Dead is close to Meat is Murder, and Strangeways, Here We Come, from what I've heard of it, is a mixed bag, though I love songs like "Girlfriend in a Coma."

Comes down to "Meat is Murder" vs. 'The Queen is Dead" with "Queen" edging out "Meat" by the smallest of margins.

I know what you mean by "Smiths proper," but I would still include "Hatful of Hollow" and "Louder Than Bombs" as albums unto themselves rather than compilations. There are a notofious number of reassembled and repackaged extraneous Smiths releases, starting with "The World Won't Listen."
However, unless you collected the singles, the material on "Hatful" and "Louder" was generally fresh when the albums were released.
In the U.S. you could buy some Smiths singles at finer urban record stores. However, to collect all the material on these albums before they were released would require an extrordinary amount of time and money, and involved special-ordering from the UK.
Nobody in the U.S. save a few college radio enthusiasts heard of the Smiths before Sire Records really started pushing "Meat Is Murder." "How Soon is Now" is the first Smiths song I heard and "Meat Is Murder" is the first Smiths records I bought. Most American Smiths fans bought the eponymous album and "Hatful of Hollow" post-Meat Is Murder.

I've gone for 'Queen'. 'Meat is Murder' is my least favourite - it's the only one I don't own, even though I still have most the tracks from it.

I know, I can't really consider myself a Smiths' fan cos' I only have the 'Singles' compilation. But they ARE one of my favourite 80s bands. Worry not, I shall listen to the albums individually when time and circumstance permits.