Critical Activity Definition

Hi All,

Somebody gave me a Construction Activity schedule but when I run it using "Total Float less than or equal to Zero (0)"  I saw no critical activity but when I changed to "Longest Path" then I saw my critical Activities in Red Colour.

Can anyone explain the best way to define critical activity and what is their pros and cons between these two?

1. Total Float less than or equal to Zero (0)    or

2. Longest Path

Thanks