In the court case of Mr. and Mrs. Smith v. Auburn Bunston Dealership, who was the plaintiff?

The answer is Mr. and Mrs. Smith.

Explanation:

In a lawsuit the plaintiff is the party or parties who are bringing the action or suing for some type of damages, usually monetary.  The defendant is the party or parties being sued, the one accused of the wrong doing.  In the style of the case the plaintiff's name comes first.  For example, Smith vs. Jones, Smith is the plaintiff and Jones is the defendant.