What do you consider the best way or tool to define scope in a project?

PMBOK tools and Techniques for Define scope have the following :-

1) Facilitated workshops are more effective than any automated tool or software. If anything the automated tool or software should be a repository of any decisions that have already come out of Facilitated workshops
Workshops are about people, bringing stakeholders together and discussing what they would like to see in the product. This is very important to engage stakeholders and make sure their requirements are considered and you have an opportunity to understand how critical this product or service is going to be for them.

2) Expert Judgement is all about touching base with someone in your company who has been a Subject Matter expert or Project Manager for a similar kind of product or service that you are implementing.

3) Product Analysis is all about analyzing the capabilities of the products that you can use in your project and to see how it fulfills your requirements

4) Alternatives Generation :- What are the different ways that you can fulfill the scope . Can you purchase something off the shelf, build something, employ someone to consult?

I consider the best way or tool to define scope in a project are:

I consider the best way to define scope in a project are:
-Interviews
-Questionnaires and surveys
- Context diagrams
-Interviews

Eduin, just to add - for the scope, things may not go as planned initially. With time many things get clear and new stakeholders get added - due to this reason defining scope looks complex.
Within the scope we need to elicit requirements - for that, we have to take inputs from stakeholders and many times we need to investigate from the documents. The selected tools and techniques dependent on many factors, for that you need to know your stakeholders, what kind of requirements you are trying to discover, who has the decision-making authority, and who has the current and updated information.

The following video may help to understand what things are involved in the elicitation and analysis: https://www.youtube.com/watch?v=MjYmGDCDi7M

In this thread, we have discussed already tools which can be used for elicitation, for the requirements elicitation, we need to map the nature of requirements which we want to discover and the source of information(Documents and stakeholders mainly)