"Brainstorming" would be my second place pick, because getting the idea is absolutely important. "Diving in" and "Finishing" are both equally important as well, but with those two all you have is a piece that might be good, but usually not incredible. That's why I say editing it until it becomes incredible is probably the most important. "Outlining" isn't always necessary. It can be useful, but it's not MOST important, so that wouldn't be my vote.
Also, I'd argue that "Sharing" isn't actually part of the creative process, but just something you might do after you're finished with the creative process. Somebody can be incredibly creative and not share any of their artwork at all.
I say it's a loose tie between outlining and sharing.
It's important to have good structure to any piece of work; without it the idea can fall apart and become impossible to understand (which is admittedly the point of some pieces). But at the same time, sharing, for me, is very important. The feedback, suggestions, the notion that what you've created is being enjoyed or discussed by other people. It can change them and how they look at the world and everything in it. Even negative responses are useful. They can help you improve on technique and drive you to better your craft.
All the other steps listed are very important as well, but I put these two at the top.
GAH! I feel conflicted about this. I want to show my picture and such to make others happy, but /I/ have the most fun working on them... *Sigh* To me the making my viewer happy is what I want. ... But I also just want to draw.... As far as actual importance goes I can't decided on that either. I can't show off something I haven't worked on, but I also don't want to show off every doodle... Maybe 'finishing' is my best option....? GAH!!! So hard! Why so hard...?!!
I'm not the type for prolonged planning, or brainstorming. I often find myself beginning with something simple; a single idea, a conversation between two characters, anything, and the story grows from there, shaping itself as it plays out. 'Past' sets itself in-stone; 'Today' is ever in-flux with the whims of the characters; 'Tomorrow' is a pipe-dream. Fingers dance on keys, the story writes itself, and in the morning, I wake up wondering where in the nine hells it all came from.