All the prereqs obviously. Also, when I recently took the MCAT, there was a lot of A&P on there so I would suggest that. Also I hadn't taken some upper level classes like advanced molecular bio (not sure what they call it at your school, but I'm sure you know what I mean) and biochem, but I definitely saw some of that on the real thing. Especially now that I'm in those classes now haha definitely kicking myself over those missed questions that I remember.
To be honest with you, a lot of the practice books like Kaplan didn't really emphasize some of the stuff I saw on my exam. They mostly had stuff covered in the prereqs, but I saw some upper level stuff too. I'm no expert, but I think any and all upper level classes will help you not only get those small details right, but also to become a better critical thinker and tie different biological concepts together.
That being said, definitely consider A&P 1 and 2, Biochem, and something in Genetics/Advanced cell bio/microbio, etc. I don't think classes like histo and immuno would be all that helpful with details but still valuable anyway for other reasons.