social networking, forums, blog commenting, etc - they are all spam as the reason you participate there is to place your link not to add value.
I definitely think article marketing is the best way. It takes the most work to write quality articles, but I think the backlink PLUS good content in the article can bring great traffic.
Most of my quality link is coming here. And I'm happy with it.. I guess I'll also try article submission this coming November.
pcz, I totally understand why you asked the question as you're smarter than most of the people and looking for quality backlinks. I'm testing a new thing now. I look for relevant websites (they might be on page 2 or page 22, it doesn't really matter as long as they're relevant) and offer them some content for a link back to my website. I got a few so far. I know it is very time consuming but in the end you get very good backlinks. I also try to exchange links with some websites in top 20 on google. I don't even go to page 3, but I'm 8th position so I have the power to negotiate now (it took me just over one year to get there though).
Dear links from article marketing really showing quality but you can't ignore social bookmarking, forum signature and blog commenting. These things are also helpful.