Longest Path and TF

definitely moutaz, longest path may not necessarily be critical path.

Assume you have a project duration of 100 days, which mean if you filter critical activities, the activities listed will have 0 float and adds upto 100 days.

Now assume that you have project duration set for 100 days but your project activities are getting finished by 90 days, then if you filter the critical path it will show none because the float will  not be zero for any of the acitivities. Now in this scenario you have to filter the longest activities so as to determine the list of activities with the longest route to 90days.

If the "Must Finish By" date is not selected, there will be no difference between longest path and Total Float less than 0, when the "Must Finish By"date is filled, there will be big difference between them, you can choose one according to your requirement.

if you do not have a projetc must finish by date you total float will always be zero on your longest path.

If you do not have a project must finish by date then you will not know when you have fallen behind schedule and by how much. 

or if you are ahead of schedule and by how much

or if you are on schedule

I would not recommend not using a project must finish by date