Oh God... I don't know if I can pick. I'll go with American Beauty/American Psycho because that's my "right now" favorite. There's no way I could do an "all time" favorite though.
Well, it really really really depends. I really do like all of them in a particular way, but the favorite one would probably be the first itself: Sounding the Seventh Trumpet.
The Queen Is Dead -- my all-time favourite Smiths song is Cemetery Gates, closely followed by Frankly, Mr. Shankly, so it makes sense to have that album as my fave.