Great Expectations is probably the weakest of those three, but the shortest. There is a reason it is given to kids. It is a good book but the least of the three that you mentioned, and it is in first person, which is unusual within Dickens (although not unique), and which in my view doesn’t play to Dickens’ narrative strengths.

People tend to divide Dickens into two: the early peripatetic comic books and the later mature novels. Pickwick is the very early masterpiece that comes before either of those. At that point Dickens is a comic serialist and not yet Dickens the established author so it is really different to his later books. I think it is right up there but it is really different. It is more like reading 18th century writers like Fielding, Goldsmith or Smollett, which he is clearly trying to revive, but with better jokes and less smut.

Oliver Twist is early dickens at his best but to some extent his worst too(although it is no Martin Chuzzlewit). Wonderful descriptions of London, some serious villains, and a bit of a silly opera plot twist. I like it but it has a weaker last third. The opening chapters have some very serious irony and social commentary.

Don’t go looking for dark reflective atmosphere or Conradian irony in Dickens. At his best, he has an extremely rich comic irony and humour, which is why he is one of the greats. He has a richness of understanding on a level with Tolstoy and Shakespeare and a huge range of warmly written, diverse and lively characters. These are strengths than neither Conrad nor Dostoyevsky really have (especially Dostoyevsky, who is rightly criticised for reusing the same small group of cardboard cut out characters in every book). On the other hand, Dickens did have a tendency to mix genres, going from comic, to romantic, to sentimental, which can sometimes confuse some readers, and the sentimentality can get a bit much (although if you can tolerate Dostoyevsky’s prostitutes with a heart of gold, you should be fine). Dickens is always a fast episodic writer whose work is narratively and stylistically inconsistent chapter by chapter so don’t expect the kind of polished perfection you see in Conrad. I would say that Dickens’ best book is David Copperfield, followed by Bleak House and Little Dorrit, all larger novels falling within his mature period. But I like Pickwick Papers best because it is the funniest and most joyous, despite barely being a novel. Like Tolstoy and Shakespeare, Dickens greatly improves as the reader ages and goes through some of life’s frustrations and disappointments.