I can see what you mean about sweet child, because of the airplay and the fact that everyone knows it (and thus thinks they know a lot about GnR) it can become annoying at times, but for me there's still no denying that it's a great song.
As for my favourites, I tend to go with either 14 Years or Breakdown - songs that are extremely powerful and meaningful but were never singles or longstanding mainstays of a live setlist. It's hard to choose though, my favourite GnR track will change depending on what mood I'm in etc.
I can't really say I have a least favourite, although I guess My World and Oh My God would be the most likely to get skipped on my iPod.