I can tell you Propel is the most suitable ORM to use with Symfony2
I should disagree. Doctrine2 is a native ORM for Symfony. It is contained in standard Symfony distribution available here: http://symfony.com/download
Symfony documentation describes working with Doctrine mostly, not Propel. So, I don't think Propel choice is too obvious. I think you should really weight the consequences. I'd prefer Doctrine.
Here is a list of options, supported by Propel bundle: http://www.propelorm.org/cookbook/symfony2/working-with-symfony2.html