Title case isn't one thing.
A title-case headline looks different in The New York Times than in an APA paper. Each style guide has its own rules. We've implemented all eight from the source manuals.
Associated Press — Newsrooms and journalism.
Chicago Manual of Style — Book publishing, humanities, business writing.
American Psychological Association — Behavioral and social-science papers.
Modern Language Association — Humanities, literature, language studies.
The New York Times — Newspaper headlines and standing heads.
Wikipedia Manual of Style — Wikipedia article titles and section headings.
The Bluebook — Legal citation, case names, law reviews.
AMA Manual of Style — Medical and scientific publication.
Which style should I use?
If your work has a specified style guide, use that one.
If you're choosing for yourself: AP for journalism and most professional writing, Chicago for books and long-form non-fiction, APA for social science papers, MLA for humanities and literature.