i need to know the most important tables to Integrate with k2

Going directly to the K2 tables is not a best practice and may even violate your license agreement.  The structure of internal tables may change between releases.  You should rely on a public API or one of the many system SmartObjects to get at this data. 

If you’re looking for process information, open the SmartObject Service Tester tool and take a look at the many system SmartObjects found under the “All SmartObjects” node.  There are several SmartObjects there that deal with process, activity, and event information.  Using these will remove your dependency from the underlying table structure.