Relevance

Two things get judged at once. Topical fit first: do the title, the description and the video itself answer what was typed. Then personal fit. Is this the sort of thing this particular viewer watches, given everything YouTube already knows about them. A video can be a perfect answer to a query and still rank below another because it failed that second test, and no amount of rewriting the title fixes that. Stuffing 30 keywords into a description only ever addressed the first half.