Which kind of backlink is best for SEO

[Steplead]

After complete the jobs of keywords, it may worth to spend more time for building up more backlinks to enhance the SEO, so do you have any suggestion about which kind of backlink is best to build? as I know now, the backlinks are classified to be: 1. profile article,  2. book mark, 3. signature, 4. wiki article,  5. blog comment etc.  I am very new for SEO, so it's welcome to give me any suggestion or comment for my opinion as above.

Thanks a lot!

-John.

[Chris.Menke]

In support of the above answers, here are some Matt Cutts videos/interview/other resources on those various types of links:

•  What is Google's view on guest blogging for links?
• How can I guest blog without it appearing as if I paid for links? 
• Link Building Is Not Illegal (or Inherently Bad) with Matt Cutts
• Matt Cutts on Blog Comments, Links & Spam: Use Your Name, Not Keywords
• 17 Types of Link Spam to Avoid

[MattAntonino]

The best backlinks for SEO, in general, are the ones you don't control. If you place the link, it's not the best.  Blog comments, not the best. Forum signature - not the best. Directories - not the best. Guest blogs - not the best.  Earning links isn't always possible or attainable but if you're after the best, the best ones are placed on sites naturally by people who love what you're doing. Everything else is just waiting to be devalued.

[Burto]

Well I am only new myself but from what I know you want to stay away from blog comments and signatures. Quality articles are an option.

Of course your best links will come from genuine sources linking to your content but if you are going to source your own links as well I think quality articles on quality blogs is your best bet.