Percabeth for sure, but I felt really bad for Rachel until she became the Oracle. She's a really underrated character, it's kind of sad how people don't like her just because she liked Percy for a while.
Percabeth, obviously. ME. D agrees! 'Mr. D rolled his eyes. "What do they say, these days, Grover? Do the children say, 'Well, duh!'?" "Y-Yes, Mr. D." "Then, well, duh!"