Most likely "why". That's the one question I have consistently abused over and over again. In learning environments, it's normal for me to run out of the privilege to ask that question before I even get anywhere.
I also ask "when" in unlimited supply as well. Then the "how" part comes pretty easily on my own accord after those two. I just have a hard time when someone tells me "how" because they usually select the one method that works for them. I'm more comfortable knowing at least 10 separate ways of doing one thing, and the significance/impact of each path. It's for *ahem* contingency planning.
"I cannot expect even my own art to provide all of the answers -- only to hope it keeps asking the right questions." -- Grace Hartigan