See What algorithm does Stack Exchange use to determine if a question may be subjective?. Here is an excerpt from the highest voted answer there by Adam Davis:
Don't use the following words in a subject title - it connotes subjective question.
Jeff posted the regex in uservoice (As of May 2, 2009):
Regex.IsMatch(s, @"\bbest\b|\bworst\b|\bhardest\b|\byour?\b|\bfavou?rite\b", RegexOptions.IgnoreCase);
This was posted in 2009, so it may or may not have changed since then.
Because ELL is a language site, these warnings will often be irrelevant. For example, a question such as, Is "Where was you?" ungrammatical? My book says it is, but I often hear native speakers say it. would probably produce the subjective question warning even though it is definitely not subjective.
The warning is intended to prevent users from asking questions such as Which dictionary do you use? or What is your favorite thesaurus?, as such questions are subjective and would probably be closed within a few minutes anyway. However, since the warning is bound to produce false positives, it will not prevent you from posting a question.