It's hard to say without knowing at least in a general sense what your issue is. If I were in your situation, the first thing I'd ask myself is "What am I trying to accomplish?" Do want to make sure a safety issue is brought to the attention of the relevant person? Do you want an acknowledgement that you received poor service (aka an apology)? Do you need a refund of miles or money that you think you're entitled to but have been unable to get? Do you want miles or money in compensation for something that happened or didn't happen? Do you want your situation to inform a possible change of policy?

Being clear on what you want, both in your own mind and in what you communicate, is key. It's much better than just venting. What you want also affects where you should direct your complaint.