You can use "favorite" feature to mark a contact as "main" or preferred:
This is per user though, it has its pluses and minuses:
- minus: you can not mark the contact as "main" for all users
- plus: one user may be dealing with person A from company X, another may consider person B as the main contact
Another option: create new field in Contacts Recordset - a checkbox - and call it "Main Contact". Enable table view and filter for that field.
