I'm checking possibility to convert CRM with Drupal.

Which is the best recommended CRM combined with Drupal?

Comments

If I understood your question correctly, you're asking what is the best CRM module for Drupal.

As far as I know, there are a few modules that provides a complete CRM experience out of the box:
CRM
http://drupal.org/project/crm
RedHen
https://drupal.org/project/redhen
CiviCRM
https://drupal.org/project/civicrm
http://sourceforge.net/project/showfiles.php?group_id=177914

Also, go through the nodes categorized with the "CRM" term as they might lead you to previous discussions and/or recipes on how to build it:
https://drupal.org/taxonomy/term/36462

Other than that, you can achieve CRM-like functionality by using various general-purpose modules, such as Views, Rules, Entity, References, Profile2, Admin Views etc.
I suggest getting familiar with these modules first, then project what kind of CRM you need and compare what can you achieve with them.

Personally, I'd go with the general-purpose modules as they allow you greater range of flexibility and you still won't have to write code; but this depends on your requirements really.

Hope this helps.