Unless the rent is subsidized or the whole property is subsidized, there is no one to complain to. This is the rent structure that the Landlord / Management has set up.
They are breaking no law.
Unless the rent is controlled by statute or unit subsidized the landlord can raise it to any amount he/she desires. There is no law containing such increases
Unfortunately, The landlord/management company are free to establish a rental agreement and prices as they wish. There are exceptions to this general rule, for example, if the housing is subsidized or some other type of Statute regulates the prices.
From the information provided, there does not appear to be any legal issue. You knew the price when you signed the Contract. It stated an increase of $15 per year and they have followed the terms of your Contract. There has not been any Breach of you Contract, in which to complain.
The mere fact that some may pay more or less than yourself is not grounds for legal action.