The Incredibles is safest but if you mean the new Marvel type superheroes most of them are 12 cert so parental guidance is needed, most of them are fine with a bit of language not to be repeated and cartoon type violence.
I watched The Fantastic Four with my two boys last week and that was safe. As mentioned above the original Superman films are U cert.
The IMDB has a parent's guide for most films, a read though that will tell you exactly how much swearing, violence, sex scenes/references are in it, even how much smoking is in it. It is quite a good check if you are unsure but it is set to max prude level!