I’m working on an application with some complex dropdowns which have selectable main categories and sub categories. Some dropdown lists could have up to 40 options - which is quite intimidating to new users of the app. Users must select a top level category but if they choose they can also select a sub for more detail.
My question is which dropdown format do you think is the most usable? Should take into account new users compared to power users and which would be more user-friendly and efficient.
A predictive search and dropdown - user can type the start of a main or sub category which would change the list to only the items beginning with those letters.
Double dropdown - User selects a main category from the first dropdown which would activate or change the second dropdown.
Multi level dropdown - User only sees one dropdown and can click (or hover) over each item to reveal the sub categories.