15. Penguin
The left-handed dagger to Panda's rapier, Penguin was another spam-fighting quality update to address the issues of poor quality linking. Although the range of ranking signals in the algorithm was already complicated (and likely getting ever more so with the improvements in search), creating a large number of backlinks was still an effective tactic to artificially boost the visibility of a website.
Penguin effectively obliterated most of the established methods of automatic link creation virtually overnight, and over successive iterations continued to target link-related spam tactics such as over-optimised anchor text, link directories, link networks and forms of paid linking which were against Google's guidelines. Sites in violation could see traffic drops as links were devalued, or find themselves getting a "nastygram" in Search Console telling them they had been penalised.
Some major brands were caught out by Penguin, and suffered major visibility loss during some key retail phases until they were reinstated. Google even devalued most of the mainstream media websites for selling advertorial links. Other popular tactics such as sponsored blog posts also died out as the links became a toxic liability.
Until Penguin was rolled into the main algorithm in 2016, sites were reliant on updates and refreshes to re-evaluate the website after clean-up activity. As these became infrequent, some sites had to operate for nearly two years under effective traffic penalisation for bad linking behaviour. Some businesses had to start over with new domains, or even shut up shop completely.
Between Panda and Penguin, the initial mission goal to clean up Google SERPs was almost a complete success. Although attempts to subvert loopholes still emerge, it is far less trouble to work within the Google recommendations on linking and content than against them.