This isn't a trick question, but it's surprising to me how many people don't know the right answer. Many will choose a specific estate planning document as the most important part of their estate plan, such as their Last Will and Testament, their Revocable Living Trust, their Durable Power of Attorney, or their Living Will. But each and every one of these documents will fail miserably without this one most important thing - naming the right person to do each of the "jobs" your estate plan will require.
You see, this is by far the most important part of your estate plan, because if you fail to choose the right person for the job, then each of the tasks required to complete the job won't be done properly, won't be done in a timely manner, or simply won't be done at all.
One of the worst things that can happen is to name someone who you think will be the right person to act as your Personal Representative, or Successor Trustee, or Attorney-in-Fact, or Healthcare Agent, or Guardian for your minor children, but when the time comes that person, for whatever reason - personal, financial, legal, political - declines to serve. You see, the people you have named as your agents in your estate plan can in fact say "Thanks, but no thanks." And then what happens if the backup person you've named also declines to serve? This means that either a judge will have to make all of the important decisions for you and your family, or the judge will have to find someone who is willing to take on the job even though that person would not have been your first choice, let alone your second choice, for the job.
Another terrible thing that can happen is when you name someone who you think will do a good job, but in the end they simply don't have the time or the skills to do the job right.
So, aside from figuring out if you need a Will or a Revocable Living Trust and who gets what and when they'll get it - which are all important decisions for creating a good estate plan - the most important thing that you will need to do when creating your estate plan is to choose your "fiduciaries" - the legal term for the people you choose to do all the "jobs" your estate plan will require - wisely.
If you're interested in learning more about this topic, then sign up for my new eCourse (in other words, a series of lessons that are delivered by email), 10 Steps to Creating a Good Estate Plan (yes, there really are 10 steps!). This particular topic is discussed in more detail in Step #7.