My son is digging Transformers. He likes the cartoon so I give him a new book once a week or so. He's 4 so he is just starting to read but, he knows the characters from the show and the movie. He likes Power Rangers so we buy him those pre-school readers they sell in Wal-Mart.
It kinda doesn't matter what you start your kid on...the best thing to do is get them interested in reading as early as possible.

This comic and an issue of Action Comics were the first comics I truly remember....sometime around 1975.
I was hooked.