That's a legitimately difficult question, especially for The Smiths.
Considering I am admittedly not a very big or devoted fan of Moz after the mid-late 90's, it's a little easier to answer the later, but still not something I can answer with one song (and I am seemingly one of the few people who actually "even" liked the albums, Kill Uncle and Southpaw Grammar... People spit on me for it sometimes).
Off the top of my head, and at this current time (it changes), these are 5 favorite Smiths tunes:
1 Wonderful Woman
2 The Hand That Rocks The Cradle
3 Still Ill (Live vers. on Rank! Too good for words)
4 A Rush And A Push And The Land Is Ours
5 Ask (the "Unreleased Bootlegs and Demos" version... It's comparatively stripped down yet still amazingly good and refreshing)
And less recently, but within the last few months:
1 I Want The One I Can't Have
2 I Want The One I Can't Have
3 I Want The One I Can't Have
4 I Want The One I Can't Have
5 The rest of MIM, but I Want The One I Can't Have in particular. Unrequited love is a bitch. It's still a bitch!
Bonus: Private "electronic music" recordings of various looped guitar passages from the studio vers. of "Ask" played backwards that I've made for my own amusement. I'm completely serious. Shit sounds great any way you spin it.
5 long-time favorite from when I first heard them:
1 Some Girls Are Bigger Than Others
2 Shakespeare's Sister
3 Miserable Lie (A lot of people trash this one, but it's probably one of the ones that REALLY got me hooked personally)
4 Basically the whole s/t and various TQID and ...Bombs tracks. This was the stuff I heard first
5 Panic (yeah, yeah... EDIT: Actually, I can recall a time when I hated this song, but that didn't last long)
I don't hate Golden Lights or Meat Is Murder, even (the later in particular is an amazing, eerie piece that I think has an undeserved bad rep. among Best Of-only fans and the generally close-minded)
I'll keep it brief for Moz and just stick to my current favorites (I could list more, but I've exhausted my enthusiasm on the previous lists):
1 I Am Hated For Loving
3 The Lazy Sunbathers
4 Our Frank
5 I Don't Mind If You Forget Me
The Hand That Rocks The Cradle is amazing! It's unique on so many levels and it's a shame Morrissey has never bothered to talk about! The lyrics.....christ, Morrissey hasn't wrote anything quite the same ever since.
And yes, I wasn't a massive fan of Viva Hate myself...but it grew on me after 3 years haha!
Do you like Bona Drag then? thats a great Morrissey album you'd love!