Help! I have no idea what AP class to take Junior year...

Depends on future career aspirations and, to a lesser degree, college choices. Don’t take a class simply to impress colleges - they won’t be impressed by a single course. Rather, they will look at the schedule as a whole.

In general, AP Physics C is geared to future physics/engineering/CS majors. A prospective premed or Russian major will find AP Physics C to be overkill.