**SPOILER ALERT** (I have warned you)
LEO AND FRANK FIGHTING OVER HAZEL WAS ADORABLE, BUT MARK OF ATHENA HAD SUCH A SAD ENDING T^T. Other than that, Mark of Athena was beautiful. In the Heroes of Olympus series, I personally liked House of Hades because it was one of the longer books in the series and I really do luxury cruise Caleo now. You can't NOT ship Caleo unless you ship Leo and Hazel. Calypso is all badass against Gaia :'D. I loved how Nico was also kind of part of the quest in House of Hades and I loved the part where Annabeth and Percy were all we-don't-care-if-you're-a-titan-or-a-giant-you-can-help-us and all stuff like that. Also the part when Hazel is being awesome against that evil sorceress. She fell into a hole that didn't exist xD.
But in the Percy Jackson series, the first five books, my favourite was Titan's Curse. I love the Hunters and Artemis. They are by far my favourite immortal people. Leo is my favourite character ^~^. After Artemis and the Hunters, it would be Calypso. Zoe should not have died, she was so epic and Bianca too D:.