I guess whatever tickles your fancy, really!
If the book is good and interesting to you personally, it will sustain your motivation when (or if) you have to work hard at every sentence and page. I usually find that the second half of a book goes much better than the first half, because I've immersed myself in the story, gotten used to the specific vocabulary used by the author, and already feel proud of how far I've come.
Reading something in French you've already read before in English is one idea. I know a lot of people have enjoyed reading Harry Potter in French despite being beginners/intermediate learners, because they already knew the story well. Some people frown on reading translations, but I think reading something you are familiar with trumps that kind of purism. Any reading practice helps!
So whatever you enjoy and can get hold of in French, basically.