What according to you is severe Black Hat Technique?

Keyword stuffing.
Crap content.

Link Cloaking

Negative SEO on your competitors.

Link Cloaking is the most severe black hat technique and anything that doesn't abide the rules of google is called black hat

People cloak links to protect their interest from Google. Link cloaking is not link masking. Cloaking is showing spiders to a different location as humans. People do it to protect themselves from devaluations that may occur by spiders discovering that their outbound links are to places link Amazon and to prevent the appearance of a thin-affiliate-site, or even to try and prevent link juice from flowing to the page they are trying to promote. What makes Google White to call an opposing will Black? No no Google is not that benevolent. They tis a business ran by men. Cloaking is a grey hat technique because it blurs the lines of who is right and who is wrong. It's only Black if Google's TOS is lord, which they are not... but they are powerful and almost so powerful that it becomes easy to assign biblical power to their TOS.

The Most severe Black Hat Techniques, imo, are a toss up between clickjacking, which can be used to fake clicks to steal money from ad programs, and Cookie Stuffing, which can be used to steal money from affiliate programs without actually promoting them.... but you can stuff cookies on pages that contain ads and reviews for those products and this grey's the practice. Negative SEO is pretty bad too.

Making thousands of backlinks using spam programs like xrumer or scrapebox.
Also doing negative SEO to your competition

Anything which exploits loopholes in the search engines is black hat. The worst of blackhat tactics would be negative SEO on your competitors sites, it's just not ethical.

That said, it is impossible to prove whether negative SEO actually works, some websites are obviously too strong for any negative SEO to affect them anyway.

Cloaking is by far the blackest of the blackest seo technique.

I would say software because it's puts a big strain on the web and causes a high server loads when it's abused. Some sites even shut their doors because of this or Google will ban them completely from their search engine or demote them to page 600.

All of it!

Cloaking etc.. But basically most "whit hat" techniques are considered black hat by google, as backlinking should be done by your visitors, not you

For me keyword stuffing, putting a long lists of keywords and nothing else onto your site will get you penalized eventually by search engines.

All the Black Hat Techniques are bad. Cant mention any one. All these technique should be avoided - Negative SEO, Cloaking, Keyword Stuffing, Hidden Text, Link Farming, Doorway Pages, Pingback Spamming.

Automated software for spamming, keyword stuffing, duplicate contents.