For NWN EE users, the current preferred version of CEP is 3.02 (CEP 3.02 The Community Expansion Pack | The Neverwinter Vault)
For NWN Diamond (1.69) users, the current version of CEP is 2.67 (CEP 2.x (Community Expansion Pack) | The Neverwinter Vault)
Currently, the CEP is divided into two forks, a CEP 3 fork maintained by Winternite, and a CEP 2 fork maintained by Proleric. As CEP 2 is primarily concerned with maintaining backward compatibility with NWN Diamond (1.69), while allowing limited access to EE features, the preferred version for EE Builders is CEP 3.01, as this will make the most usage of the new features incorporated into the EE (PBR maps, new script commands, etc.). CEP 2 is more for Players who wish to play modules constructed with CEP 2 that have not been upgraded to CEP 3.
For Builders wishing to upgrade from CEP 2 to CEP 3, upgrading is simply a matter of changing the module's HAK list and associated TLK under the module's Custom Content tab according to the instructions provided on the CEP 3 Project page (linked above). Contrary to myths being circulated around the community, CEP 3 will NOT break your CEP 2 module. All content included in CEP 2 is included in CEP 3.