Every ScrumMaster has a favorite practice that is supposed to be the most important one (all practices are of course equal in importance, but insert the favorite here is more equal).
For a long time I was hesitating between Daily Standups and Sprint Planning Session, but my final choice (for today at least) is the Sprint Retrospective Meeting, a.k.a. the Iteration Post Mortem (sorry for remains of corporate speech ;-))
Sprint Retrospective Meeting is meant to provide the opportunity to identify positive and more importantly negative factors your team has recently encountered, prioritize them with respect to impact on team performance and take measures to promote desired ones and remove or at least contain the negative ones.
If your team has such meetings regularly, everybody participates actively and then holds to the decisions made, all other practices will be in place soon. If they would benefit the team.
On the other hand, if your team is unable to implement Sprint Retrospectives, what are the chances that it is able to properly implement other Scrum practices? Or just any practices?