What is the highest GPA for AP classes?

There's no single "highest GPA for AP classes." A GPA is a calculation of your grades across all your classes, not just AP classes. While an individual AP class might be weighted (giving a higher GPA point value than a non-AP class), the maximum GPA achievable depends entirely on the school's weighting system and grading scale. Some schools might use a 4.0 scale, others a 5.0 or even higher, with different weights applied to AP classes.