If it's someone I don't know I'll just respond I'm alright, but that is mostly a brush off because I don't actually want to talk to them. If I know someone well I try not to lie. If I'm doing well that day I'll respond "better than usual" and if I'm not doing well I'll respond "same as usual". Other people don't want to hear that you are down, because it will bring them down with you. The concept of asking someone how they are doing, I feel is a bit self serving in that regard. If you say " not great" They'll respond "I'm sorry to hear that" and then the mood becomes less healthy. If something specific is actually bothering you, you could say "I'm doing well except for this one thing" if its something you actually want the person to know about, or you want to talk about it. That way thr mood is positive, and the person is more willing to listen.

Now if the greeting is "What's Up?" I will Always respond the cieling, or clouds if I'm outside, because I'm a sarcastic asshole.