I have lots of project sites each with their own task list. I want to create a master task list (or view or whatever?) on the parent site that can display all tasks in all of these project task lists. ideally it can then be filtered too. I know its possible but just wanted to find the easiest most effective way of doing this. Thanks
The easiest way to create a master task list is by using a custom display template. So, you can place a content search web part on the page you would like, and in the query of the CSWP, you can configure to return only items that are using the task content type (I usually create a custom content type that all my tasks list use - keep a note of its content type id).
Here is a good resource on how to create a custom display template (if you don't have experience creating one)
Try using basic workflow to create new items in a master list each time an item is created in one of the separate ones. I think it might be better to do it this way so you can put in meta data columns for each one of your separate list into the master. You would make a workflow on each of the separate list to do this in SharePoint designer. Pretty straight forward. Then you can make all of the views you want based off of the different list information in the master.