I'd go for "The Mercy Seat", although there are many other possibilities - Cave's standards and consistency are remarkably high (was listening to "The Boatman's Call" on the way home today).

I really like his and P.J. Harvey's version of Henry Lee.

And raise you with From Her To Eternity.

I never really got Nick Cave until I saw the Bad Seeds play the song years ago at a festival during a thunderstorm - where it absolutely blew me away.

(Badly truncated clip taken from Wings of Desire)

I've never seen him live but I hear talk of a big tour when the new bad seeds album comes out next year and I'll be there.

Re: From Her To Eternity, I'm not actually that hot on the old stuff, I've been working backwards through their albums and I only got to The Good Son today and I downloaded Tender Pray at the same time, at my current rate I'll reach From Her To Eternity some time in 2008.

This song works as a good litmus test. It's Cave in flat-out sweary, scary mode, but with his tongue rammed so far up his cheek it's damn near pokin' his eye out. Anyone who blanches at the repeated Oedipal compound noun, or the bloody violence, or who can't raise a chuckle somewhere along the way, probably won't ever fully snuggle up to Mr.Cave's oeuvre. Myself, I think it's a hoot. "Murder Ballads" is Cave's funniest album.

I think "Henry's Dream" as an album is a good entry point, despite Cave hating the production on the whole thing. And if I had to pick one track from it I'd go for either "Papa Won't Leave You Henry" or "John Finn's Wife".
Both epitomise the many Cave songs that in certain moods you might take seriously, and in other moods are a real hoot (especially the ending to "John Finn's Wife"). Like Morrissey and Tom Waits, it's often the so-called gloom-mongers (the Nabobs of Sob, the Princes of Wails, etc) who can actually be the most fun.

If someone wanted to wade deep into Cave's messed-up mid-Eighties mind, "Your Funeral... My Trial" is as otherworldly as it gets. "The Carny" would probably get my vote from that album as an advert for Cave, although "Sad Waters" is probably my favourite.

[NB. You probably knew, but in case you didn't - "Mr Cave's A Window Cleaner Now" is a Half Man Half Biscuit song. "Release the pigeons!"].

...on the grounds that the Abattoir Blues/Lyre Of Orpheus set is his best work to date, I nominate my favourite track: Get Ready For Love.