"Doing well". Of course, no matter how I feel like I'm really doing. It's upbeat (as i emphasize in my tone), which is the real point. This is almost always a question I get in professional settings and communicating upbeatness is important. :)

Plus, it's in a sense, always true. I'm not dying of some horrible disease (or dead), starving and on the streets, and live in the land of milk and honey. So, no matter how despondant something has me on that particular day, it's hard to argue that in the bigger picture, I'm not doing well. :)