In a sense, a discussion of the roles of Iago and Othello in William Shakespeare's Othello is really one of literary terminology. The terms "hero" and "villain" assess not the importance of the roles but the moral quality of the characters.
A set of more technical terms used by theatre critics would be "protagonist" (derived from a Greek word meaning first contests) and "antagonist" (from a Greek word meaning opponent). The protagonist is the leading character in the play, and also one whose fate is the main subject of the play. The antagonist is the main opponent.
Othello is the protagonist because the story revolves around something that happens to him and changes him (from loving husband and successful officer to madman and murderer). Iago, although tremendously important, as you mention, is a static character who does not develop over the course of the drama. His role is that of antagonist.