I got a way worse grade than my classmates for equal performance. What should I do?
Easy answer:  Go talk to your professor at office hours.  You should escalate to your department chair or the dean only after you've tried in good faith and failed to resolve the problem directly with your instructor.  Don't escalate unless it's genuinely serious.

Try to be open-minded toward the possibility that there were deficiencies in your work that you may not have spotted that explain why your grade was lower than you think it should be.  Your fellow students' opinions are not reliable indicators of anything nor is it helpful to cite their complaints or opinions as justification for yours.  Go with the expectation you will learn something, not with the sole goal of getting more points.