I would defenatly say Dedication, having to pick only one, but following Dedication I would most certanly say Imagination.
I think dedication is most important because I have seen many a times people having most of the other stuff that you listed, yet not being satisfied with their results, expecting that the knowledge of LDing they have will automaticly give them a LD every night, thus leading them to not explore tehniques, practice and similar aswell as people giving up after a few unsuccsesfull tries. From my personal expiriences, best progress I had when I gave full dedication and emmersed myself fully in the dream world, when I set up taska, schedules and goals that I follow. As for Imagination, I think it is important aswell, to induce curiosity and fantasy that helps a lot with getting interested in expirimenting with LD, practicing and so forth.

*I hope you do not mind me answering to your topic, seeing that I am a new member