I've read both of those; Papillon is fascinating (though, according to French friends, maybe not entirely reliable or accurate. Seemingly, while a lot of what is depicted in the book happened, it didn't all necessarily happen to Papillon himself......Who knows? Perhaps they are biassed.)
The Count of Monte Christo has one of the best opening few hundred pages that I have ever read. Like Dickens, Dumas wrote many of his books - and initially, they were published as such - originally in serialised form in magazines (the completed book would be published subsequently). This meant that it was in their interest to pad out a story for a variety of reasons - and it also means, to my mind, that both of them could have done with the services of a good sub-editor. But the first few hundred pages (until Dantes escapes) are as gripping as anything I ever read as a kid.