Are the ones you make yourself. They are the ones that are quality, and have effort put into them. Therefore, they most likely do not get deleted. There are lots of good ways to find places to make potent backlinks. I have been using this method personally for the past couple of days putting in manual work on some backlinks. Use footprints to find places that you can add a backlink. Such as: "keyword" + "powered by vbulletin". The blogs you can find using that footprint is awesome. It is basically the best type of blog comments you can get. Most of the blogs I found have anti-bot questions, to prevent bots from spamming. These are awesome captchas for humans because they ask questions that only humans would know. Spend a hour or so just registering and adding blog comments to all of the results, and every day you will see you have 15-20 targetted backlinks. The links you can find are also 100% keyword targetted, because it already has that content. I just register to each site, write a 100 word article on each one of the blog comments, and throw my anchor links in there. Thus, it is quality content that has 100% readability, and they will stick. Then blast the shit out of those with xrumer and scrapebox. And viola, you have made your own good links.