Called patent trolls or non-practising entities (NPE), companies that make most of their money from licensing patents don't have the best reputation.

Just this week, the Wall Street Journal reported U.S. antitrust authorities are probing whether these firms are making high-tech markets less competitive.

Indeed, many tech companies live in fear of NPEs.

It's tough to quantify which NPEs are the most feared, since many NPE's simply threaten to sue companies, patent expert James Bessen told Business Insider.

The scariest NPEs, however, tend to be the ones with the biggest patent arsenals, Bessen said. Those companies typically also have the resources to file suit if they want.

With that in mind, BI ranked NPEs based on their number of patents and publicized patent applications as of July 2012 reported by PatentFreedom, a group that provides research for patent defendants.

Note: PatentFreedom defines an NPE as any company that derives the majority of its income from licensing patents. Some of the NPEs listed, like Tessera, also invest in research and development.

BI reached out to every ranked company for which contact information was available.

