By Anne Fisher
August 15, 2012

FORTUNE — Brain-teaser questions, timed written tests that rival the GMAT, successive rounds of rapid-fire interview sessions with intensely focused hiring managers — are you ready for all these, plus the occasional odd moment of catch-you-off-your-guard eccentricity?

Career site sifted through more than 80,000 job hunters’ interview ratings and reviews over the past 12 months to come up with this list of the 25 companies where getting hired is hardest. The number at the right is each company’s difficulty rating on a 5-point scale where 1 is “very easy” and 5 is “extremely difficult.”

1. McKinsey & Co. – 3.9
2. BCG (Boston Consulting Group) – 3.8
3. Oliver Wyman – 3.7
4. A.T. Kearney – 3.7
5. ZS Associates – 3.7
6. Thoughtworks – 3.6
7. Bain & Co. – 3.6
8. Royal Dutch Shell (rdsa) – 3.6
9. Google (goog) – 3.5
10. Unisys – 3.5
11. Rackspace Hosting – 3.4
12. Cypress Semiconductor – 3.4
13. Susquehanna International Group – 3.4
14. BazaarVoice – 3.4
15. P&G (pg) – 3.4
16. Teach for America – 3.4
17. L.E.K. Consulting – 3.4
18. Juniper Networks (jnpr) – 3.4
19. Sapient (sape) – 3.4
20. Stryker (syk) – 3.3
21. General Mills (gis) – 3.3
22. Progressive (pgr) – 3.3
23. Headstrong – 3.3
24. Facebook – 3.3
25. Amazon (amzn) – 3.3

It’s no surprise that so many of the most challenging job interviews take place at consulting firms. After all, these companies’ only product is brainpower. So their interviewers are partial to posing knotty questions like “How many people would use a drug that prevents baldness?” (BCG) or “What is the profit potential of offering wireless Internet service on airplanes?” (Oliver Wyman). At McKinsey, candidates must also take a written quiz loaded with charts and figures that has to be “analyzed swiftly with an acute sense of numbers,” one aspiring consultant told Glassdoor.

Want a job at Facebook (FB)? “Be ready to give great answers to ‘Why Facebook?’” advises a recently hired software engineer. “All seven interviewers asked me this, and it’s really important to them.”

Fair enough, but some interviewers throw candidates a curve ball, apparently just to see how they’ll react. One applicant at Boston Consulting Group reports that, in his second interview, his interlocutor “started by putting his feet on the desk (with no socks) and eating out of a bowl of soup, talking simultaneously. Odd start.” Indeed.

Among the most intriguing of Glassdoor’s findings: Most veterans of tough interviews at these 25 companies rated the experience a positive one. More striking still, the companies with the highest difficulty ratings also score highest in employee satisfaction.

That makes sense, says Rusty Rueff, a member of Glassdoor’s board of directors and the site’s resident career expert. “A company that has stringent standards for performance will really put you through your paces [in interviews] because that is an honest and true reflection of their culture,” says Rueff, a former vice president of international human resources at PepsiCo (pep). “So the people they select are the ones who thrive on difficulty — and, for people who don’t, a tough screening process gives them the chance to opt out.”

At any company, he adds, “you really need job interviews to be the best possible window into what the company is about, and what it will really be like to work there. Otherwise, you end up hiring people who won’t fit in, and you’ll end up having to replace them when they quit.”

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