The Last Olympian would be my clear favorite. How Rick Riordan concludes all the plot lines and ties the whole story together really satisfied me. The budding romance between Percy and Annabeth was a clear fan favorite and I was thrilled when it finally kicked off. Also, the ending was not too closed off and left room for future sequels which was something I was definitely looking forward to.
Obviously - The Battle of the Labyrinth
Arguably the book where Kronos was finally brought out from Tartarus.
The last olympian. All is revealed in the last book.
Titan's curse or sea of monsters!
Percy Jackson is an amazing book series and I think you should definitely start from the first book if u havent read them yet!!
but i'd have to say the Titan's Curse is the best because I honestly root for romance and humour and the book got both down.
the main characters, percy and annabeth, starts to like each other which i'm pretty sure was something every reader was waiting for!!
also, if u love tat series, i'd also recommend other book wrote by rick riordan (the author) because most of his other series hav the same basic idea of going into myths and greek stuff.
u made a great choice to read the percy jackson series!!