Its up to you. To get the most hits you want to be reviewing new films that are either just out at the cinema or on DVD.
Of course some films are easier and better to review than others. Your average comedys etc dont really offer much to write about in depth. If your going to be writing regularly then its fine to mix things up, review a multitude of films and have varying sized reviews.
But if like me you dont write as often its best to choose either big event movies that many will be interested in, or simply films where you have lots to talk about. e.g Inception (plot, performances, Nolan as Director and the film generally on a technical level. lots more to say than with Cats and Dogs 2)