Which is more selective, Williams or Skidmore?

Williams is more selective than Skidmore. In practice, Williams sits in the very top tier of liberal arts college admissions, while Skidmore is selective but noticeably less difficult to get into. If you are comparing how hard admission is on a broad level, Williams is the tougher admit.

That usually matters most for students aiming at extremely small, highly academic colleges with a national applicant pool and a reputation that draws a very large number of top students. Williams tends to attract applicants with standout grades, rigorous coursework, and strong extracurricular distinction, so even very qualified students are often competing in a much tighter process.

Skidmore fits a somewhat different applicant profile. It is still a respected liberal arts college with serious academics, but it tends to be a more accessible option for students who want a strong residential college experience, a creative or arts-friendly atmosphere, and selective admissions without quite the same level of intensity as Williams.

So if your question is strictly about selectivity, the answer is Williams. If you are building a list, Williams should usually be treated as a higher-reach school, while Skidmore is more likely to fall into a less extreme category for the same applicant.