I think I have to go with 'Still Ill' on this one. Although my particular interest in that song has waned somewhat since I first heard it (probaly due to the excessive repeated listens), it does hold a special place in my heart. For one thing, the music and the lyrics blend perfectly; it has a pounding, relentless rythm, and the opening lyrics really grab you by the balls. Those are some of the most touching, profound and heartachingly personal lyrics that Morrissey ever commited to song in the whole of 'The Smith's' libaraby in my opinion. If you'd allow me to a dial'a'cliche on this one, those opening lines articulated for me the exact feelings I had of the time in which I first listened to the song. The repeated lines: 'Under the iron-bridge, we kissed, and even though I ended up with sore lips'...are simply beautiful.
Following not to far behind would be, 'Boy with a thorn...' and 'That Joke isn't Funny...'.