Statements on a radio talk show that described the plaintiff as a "chicken butt," "local loser" and "big skank" were not defamatory because they were "too vague to be capable of being proven true or false" and had "no generally accepted meaning." Seelig v. Infinity Broadcasting, 97 Cal. App. 4th 798 (Cal. Ct. App. 2002).
A cartoon of a noted evangelist leader fornicating drunk in an outhouse with his mother because the parody was so outrageous it could not "reasonably be understood as describing actual facts" about Falwell or events in which he participated. Hustler Magazine v. Falwell, 485 U.S. 46, 53 (U.S. 1988).
Keep in mind, however, that you can't make a statement an opinion merely by prefacing it with "in my opinion." Saying that "in my opinion, Alex stole ten dollars from the church collection basket" would lead most listeners to conclude you had evidence that Alex had indeed stolen the money, and that you intend the statement as one of fact rather than opinion. The courts do not give protection to false factual connotations disguised as opinions.
Context and the Totality of the Circumstances
In general, courts will look at the context and medium in which the alleged defamation occurred. For example, a statement is more likely to be regarded as an opinion rather than a fact if it occurs in an editorial blog as opposed to a piece of investigative journalism. The wider context may also provide a framework for the court: during the McCarthy-era witch hunts of the 1950s, for example, courts routinely held that referring to someone as a "Communist" was defamatory; in the present day, "communist" has taken on a more generalized (if still often derogatory) political meaning, and courts would almost certainly find use of the word to be a protected opinion.