As a college student, I have found that Istudiez is the planner app that is best geared towards students. Assignments can be simply and quickly input, and reminders easily set. It organizes assignments, tests, quizzes…. by classes and by semester, and everything is well color coded.
This is an Apple product only thing I believe, but syncs perfectly between my MacBook, iPhone, and iPad. The calendar in it also can sync with some third party calendars, like Google Calendar and Outlook.
That being said, as an ADHD college student, the best one is whatever one actually gets used. I have had planners forced on me my whole unsuccessfully, the only way I ever started using one was because it was my own idea and priority, at age 21.
Hope that helps