I was far more impressed with the execution of Book III than I was the previous two. Perhaps it had the momentum many conclusions garner to guide it home, perhaps it was just the result of Collins having more writing practice- who knows. Much deeper concepts are explored and a level of maturity is demonstrated by the protagonists which was sorely lacking in the previous installments.
The message of oppression and rebellion is driven home. Beyond this, Collins delves even deeper into these themes, touching on the true nature of power and control. The reality that any government is bound to be oppressive to some extent. Or, as the song goes, meet the new boss, same as the old boss.
The best of the three, absolutely worth a listen if you have made it through the first two. I don't think I would listen to the previous books just to get here for a standing start though- there are much better listens out there that tackle these concepts in a far more delicate and meaningful way.