I re-read all of the Percy Jackson books (including the Heroes of Olympus ones). I remember quite liking all of them when I was 11 so it was weird when I found that only two (out of ten) of the books were any good. The Lightning Thief was an 8/10, and The Titan’s Curse was 7/10. Apart from that, they were all pretty rubbish. If I were to rank them (my favourite hobby!) it would go like this:

The Lightning Thief (8/10)
The Titan’s Curse (7/10)
The Lost Hero (5.5/10)
The Battle of the Labyrinth (5/10)
The Mark of Athena (4/10)
The Son of Neptune (4/10)
The Sea of Monsters (3.5/10)
The House of Hades (3/10)
The Last Olympian (2.5/10)
The Blood of Olympus (2/10)