There is an ongoing debate in the agile/scrum community about 'what aspect of scrum is most important'.
Obviously, the most *recognizable* aspect of scrum is the daily stand-up meeting. But - is this meeting so important? I've seen and ran Scrum teams that skipped the daily standup, had a daily standup meeting twice a week, or otherwise modified the ritual. And they worked well.
In my opinion, the two most important concepts in scrum (which are general Agile concepts), are *timeBoxing* and *adaptive iteration*. This is why I think the Sprint Planning meeting is the most crucial - this is when planning ofr the next iteration, taking into account changes that need to be made, takes place.
It is true that much of the 'lessons learned' are acquired in the *sprint retrospective* meeting, if it is held. But without incorporating this information in the next sprint planning session, the retrospective degrades to nothing but a 'whine session.